diff --git a/resources/css/humhub.mail.css b/resources/css/humhub.mail.css
index cf26cee..5bc82e9 100644
--- a/resources/css/humhub.mail.css
+++ b/resources/css/humhub.mail.css
@@ -209,6 +209,9 @@
 .mail-message-form {
   width: 100%;
   padding: 10px;
+  box-shadow: none;
+  border: none;
+  margin-bottom: 0 !important;
 }
 .mail-message-form .content-create-input-group {
   position: relative;
diff --git a/resources/css/humhub.mail.less b/resources/css/humhub.mail.less
index 9c7766b..e3df14f 100644
--- a/resources/css/humhub.mail.less
+++ b/resources/css/humhub.mail.less
@@ -11,8 +11,8 @@
 @screen-md: 991px;
 
 :root {
-  --hh-mail-offset-top: 0px;
-  --hh-mail-top-conversation-height: 0px; // Conversation is displayed at the left side of the screen
+  --hh-mail-offset-top: ~"0px"; // Force keeping `px` after CSS compilation (required for `calc()` function)
+  --hh-mail-top-conversation-height: ~"0px"; // Conversation is displayed at the left side of the screen
 }
 
 @media (max-width: @screen-md) {
@@ -295,6 +295,9 @@
 .mail-message-form {
   width: 100%;
   padding: 10px;
+  box-shadow: none;
+  border: none;
+  margin-bottom: 0 !important;
 
   .content-create-input-group {
     position: relative;
diff --git a/resources/css/humhub.mail.min.css b/resources/css/humhub.mail.min.css
index 9df5c9b..2b3e47e 100644
--- a/resources/css/humhub.mail.min.css
+++ b/resources/css/humhub.mail.min.css
@@ -1 +1 @@
-:root{--hh-mail-offset-top:0px;--hh-mail-top-conversation-height:0px}@media (max-width:991px){:root{--hh-mail-top-conversation-height:60px}}@media (max-width:767px){:root{--hh-mail-top-conversation-height:50px}}.placeholder{color:#77777a}#dropdown-messages .dropdown-header{color:#000;margin-bottom:12px;font-weight:600}#dropdown-messages.dropdown-menu li a{font-size:inherit!important}#dropdown-messages .media-body{font-weight:200}.modal-body #createmessage-message .ProseMirror{min-height:100px!important}#create-message-button .fa,#mail-conversation-create-button .fa{margin:0}#conversation-filter-link{padding-left:0;display:block;padding-bottom:0}#conversation-tags-root{border-bottom:1px solid #eee}#conversation-tags-root .my-tags-label{padding-right:10px}.conversation-menu{display:none}.conversation-menu .time{color:inherit;text-transform:none}.conversation-menu .badge{background-color:var(--background-color-secondary);border-radius:10px;color:inherit}.conversation-menu .conversation-edit-button{background-color:var(--background-color-secondary);border-radius:50%;color:inherit}.conversation-menu .conversation-edit-button:hover{background-color:var(--background-color-secondary);color:inherit}.visible-xs .conversation-menu{padding-left:35px}.visible-xs .conversation-menu .conversation-menu-item{margin-bottom:10px}.conversation-menu-item{margin-left:5px}.conversation-menu-item a{cursor:pointer}.conversation-edit-button{border-bottom-right-radius:0;border-top-right-radius:0}.conversation-scroll-down-button{position:absolute;bottom:58px;right:25px;width:38px;height:38px;background:#fff;border-radius:50%;box-shadow:1px 1px 2px #999}.conversation-scroll-down-button .fa{font-size:26px;margin:7px 0 0 11px}.conversation-entry-list{min-height:150px}.conversation-entry-list .media{padding-right:5px}.conversation-entry-content{display:table;float:left;background-color:var(--background-color-secondary);border-radius:10px;padding:10px;max-width:70%}.conversation-entry-content pre{max-width:485px}.conversation-entry-content.own{float:right;background:var(--background-color-highlight)}.conversation-entry-content .markdown-render{float:left;width:100%;min-width:230px}.conversation-blocked-recipient{-webkit-filter:grayscale(100%);filter:grayscale(100%)}#mail-conversation-root hr{border-top:1px solid #eee}#mail-conversation-root .ProsemirrorEditor.focusMenu .ProseMirror-menubar{margin-top:0}#mail-conversation-root>.panel{display:flex;flex-direction:column;height:calc(100vh - var(--hh-fixed-header-height) - var(--hh-fixed-footer-height) - var(--hh-mail-top-conversation-height) - 10px)}#mail-conversation-root>.panel>.conversation-entry-container{flex:1;min-height:300px;overflow:auto;margin-bottom:0}#mail-conversation-header{padding:18px;border-bottom:1px solid var(--background3);border-bottom-left-radius:0;border-bottom-right-radius:0}#mail-conversation-header h1{font-weight:600;font-size:22px;display:inline-block}#mail-conversation-header small{display:block;font-size:11px;font-weight:400}#mail-conversation-header small a{color:var(--text-color-main)}#conversation-settings-button{padding:6px;font-size:12px}#mail-filter-root{margin-top:5px}.mail-inbox-messages .panel-heading>a{font-weight:600}.mail-inbox-messages .panel-heading #mail-filter-root>a{font-weight:400}.mail-inbox-messages .media{margin-top:0;display:flex}.mail-inbox-messages .media h4.media-heading{font-weight:600;font-size:16px}.mail-inbox-messages .media h4.media-heading time{font-size:11px!important;font-weight:400;float:right}.mail-inbox-messages .media h5{font-size:14px;line-height:16px;font-weight:500;color:#000;display:flex;justify-content:space-between;margin:7px 0}.mail-inbox-messages .media h5 span:first-child{overflow:hidden;text-overflow:ellipsis;white-space:nowrap}.mail-inbox-messages .media .mail-last-entry{font-size:14px;font-weight:500;color:var(--text-color-secondary)}.mail-message-form{width:100%;padding:10px}.mail-message-form .content-create-input-group{position:relative}.mail-message-form .humhub-ui-richtext{padding-right:105px;border-radius:10px}.mail-message-form .upload-buttons{position:absolute;right:6px;bottom:6px;z-index:500}.mail-message-form .reply-button{margin-left:0!important}.mail-message-form .help-block{margin:0}.mail-message-form .alert{margin-bottom:0}.mail-conversation-entry{margin-top:3px;position:relative}.mail-conversation-entry .author-image .img-user{max-width:100%}.mail-conversation-entry .author-label{font-size:10px;font-weight:600}.mail-conversation-entry.hideUserInfo .author-label{display:none}.mail-conversation-entry.hideUserInfo .author-image a{visibility:hidden;margin-top:0}.mail-conversation-entry.own .conversation-menu-item{float:right;margin-left:0;margin-right:5px}.mail-conversation-entry .conversation-entry-time{float:right;font-size:10px;color:var(--text-color-main)}.mail-conversation-entry .conversation-entry-time>span{font-style:italic}.conversation-entry-badge{padding:10px 0 8px;text-align:center}.conversation-entry-badge span{display:inline-block;box-shadow:1px 1px 3px rgba(0,0,0,.3);border-radius:7px;padding:5px 20px}.conversation-entry-badge.conversation-date-badge span{text-transform:uppercase}#inbox{max-height:calc(100vh - var(--hh-fixed-header-height) - var(--hh-fixed-footer-height) - var('--hh-mail-offset-top'));overflow:auto;border-radius:4px}.messagePreviewEntry{cursor:pointer}.messagePreviewEntry time{font-size:10px;color:var(--text-color-secondary)}.messagePreviewEntry .new-message-badge{display:none}.messagePreviewEntry.unread .new-message-badge{display:block}.messagePreviewEntry.unread time{color:var(--info)}.messagePreviewEntry.unread .mail-last-entry{color:var(--text-color-highlight)}.message-tag-filter-group .select2-selection{border-bottom-right-radius:0}.message-tag-filter-group .manage-tags-link{font-weight:600;font-size:.8em;border:1px solid #ededed;border-top:0;border-bottom-left-radius:4px;border-bottom-right-radius:4px;padding:2px 5px}.new-message-badge{float:right;min-width:14px;height:14px;border-radius:50%;background:var(--info);margin-left:2px}.inbox-entry-title{font-weight:600}.field-replyform-message{margin:0}@media (max-width:991px){#inbox{max-height:500px!important}}@media (max-width:767px){#dropdown-messages{width:300px!important}.arrow{margin-left:-101px!important}#inbox{max-height:none!important}.mail-conversation-single-message #inbox{max-height:none!important}.mail-conversation-single-message .inbox-wrapper{display:none}.conversation-entry-content pre{max-width:245px;padding:0}}
\ No newline at end of file
+:root{--hh-mail-offset-top:0px;--hh-mail-top-conversation-height:0px}@media (max-width:991px){:root{--hh-mail-top-conversation-height:60px}}@media (max-width:767px){:root{--hh-mail-top-conversation-height:50px}}.placeholder{color:#77777a}#dropdown-messages .dropdown-header{color:#000;margin-bottom:12px;font-weight:600}#dropdown-messages.dropdown-menu li a{font-size:inherit!important}#dropdown-messages .media-body{font-weight:200}.modal-body #createmessage-message .ProseMirror{min-height:100px!important}#create-message-button .fa,#mail-conversation-create-button .fa{margin:0}#conversation-filter-link{padding-left:0;display:block;padding-bottom:0}#conversation-tags-root{border-bottom:1px solid #eee}#conversation-tags-root .my-tags-label{padding-right:10px}.conversation-menu{display:none}.conversation-menu .time{color:inherit;text-transform:none}.conversation-menu .badge{background-color:var(--background-color-secondary);border-radius:10px;color:inherit}.conversation-menu .conversation-edit-button{background-color:var(--background-color-secondary);border-radius:50%;color:inherit}.conversation-menu .conversation-edit-button:hover{background-color:var(--background-color-secondary);color:inherit}.visible-xs .conversation-menu{padding-left:35px}.visible-xs .conversation-menu .conversation-menu-item{margin-bottom:10px}.conversation-menu-item{margin-left:5px}.conversation-menu-item a{cursor:pointer}.conversation-edit-button{border-bottom-right-radius:0;border-top-right-radius:0}.conversation-scroll-down-button{position:absolute;bottom:58px;right:25px;width:38px;height:38px;background:#fff;border-radius:50%;box-shadow:1px 1px 2px #999}.conversation-scroll-down-button .fa{font-size:26px;margin:7px 0 0 11px}.conversation-entry-list{min-height:150px}.conversation-entry-list .media{padding-right:5px}.conversation-entry-content{display:table;float:left;background-color:var(--background-color-secondary);border-radius:10px;padding:10px;max-width:70%}.conversation-entry-content pre{max-width:485px}.conversation-entry-content.own{float:right;background:var(--background-color-highlight)}.conversation-entry-content .markdown-render{float:left;width:100%;min-width:230px}.conversation-blocked-recipient{-webkit-filter:grayscale(100%);filter:grayscale(100%)}#mail-conversation-root hr{border-top:1px solid #eee}#mail-conversation-root .ProsemirrorEditor.focusMenu .ProseMirror-menubar{margin-top:0}#mail-conversation-root>.panel{display:flex;flex-direction:column;height:calc(100vh - var(--hh-fixed-header-height) - var(--hh-fixed-footer-height) - var(--hh-mail-top-conversation-height) - 10px)}#mail-conversation-root>.panel>.conversation-entry-container{flex:1;min-height:300px;overflow:auto;margin-bottom:0}#mail-conversation-header{padding:18px;border-bottom:1px solid var(--background3);border-bottom-left-radius:0;border-bottom-right-radius:0}#mail-conversation-header h1{font-weight:600;font-size:22px;display:inline-block}#mail-conversation-header small{display:block;font-size:11px;font-weight:400}#mail-conversation-header small a{color:var(--text-color-main)}#conversation-settings-button{padding:6px;font-size:12px}#mail-filter-root{margin-top:5px}.mail-inbox-messages .panel-heading>a{font-weight:600}.mail-inbox-messages .panel-heading #mail-filter-root>a{font-weight:400}.mail-inbox-messages .media{margin-top:0;display:flex}.mail-inbox-messages .media h4.media-heading{font-weight:600;font-size:16px}.mail-inbox-messages .media h4.media-heading time{font-size:11px!important;font-weight:400;float:right}.mail-inbox-messages .media h5{font-size:14px;line-height:16px;font-weight:500;color:#000;display:flex;justify-content:space-between;margin:7px 0}.mail-inbox-messages .media h5 span:first-child{overflow:hidden;text-overflow:ellipsis;white-space:nowrap}.mail-inbox-messages .media .mail-last-entry{font-size:14px;font-weight:500;color:var(--text-color-secondary)}.mail-message-form{width:100%;padding:10px;box-shadow:none;border:none;margin-bottom:0!important}.mail-message-form .content-create-input-group{position:relative}.mail-message-form .humhub-ui-richtext{padding-right:105px;border-radius:10px}.mail-message-form .upload-buttons{position:absolute;right:6px;bottom:6px;z-index:500}.mail-message-form .reply-button{margin-left:0!important}.mail-message-form .help-block{margin:0}.mail-message-form .alert{margin-bottom:0}.mail-conversation-entry{margin-top:3px;position:relative}.mail-conversation-entry .author-image .img-user{max-width:100%}.mail-conversation-entry .author-label{font-size:10px;font-weight:600}.mail-conversation-entry.hideUserInfo .author-label{display:none}.mail-conversation-entry.hideUserInfo .author-image a{visibility:hidden;margin-top:0}.mail-conversation-entry.own .conversation-menu-item{float:right;margin-left:0;margin-right:5px}.mail-conversation-entry .conversation-entry-time{float:right;font-size:10px;color:var(--text-color-main)}.mail-conversation-entry .conversation-entry-time>span{font-style:italic}.conversation-entry-badge{padding:10px 0 8px;text-align:center}.conversation-entry-badge span{display:inline-block;box-shadow:1px 1px 3px rgba(0,0,0,.3);border-radius:7px;padding:5px 20px}.conversation-entry-badge.conversation-date-badge span{text-transform:uppercase}#inbox{max-height:calc(100vh - var(--hh-fixed-header-height) - var(--hh-fixed-footer-height) - var('--hh-mail-offset-top'));overflow:auto;border-radius:4px}.messagePreviewEntry{cursor:pointer}.messagePreviewEntry time{font-size:10px;color:var(--text-color-secondary)}.messagePreviewEntry .new-message-badge{display:none}.messagePreviewEntry.unread .new-message-badge{display:block}.messagePreviewEntry.unread time{color:var(--info)}.messagePreviewEntry.unread .mail-last-entry{color:var(--text-color-highlight)}.message-tag-filter-group .select2-selection{border-bottom-right-radius:0}.message-tag-filter-group .manage-tags-link{font-weight:600;font-size:.8em;border:1px solid #ededed;border-top:0;border-bottom-left-radius:4px;border-bottom-right-radius:4px;padding:2px 5px}.new-message-badge{float:right;min-width:14px;height:14px;border-radius:50%;background:var(--info);margin-left:2px}.inbox-entry-title{font-weight:600}.field-replyform-message{margin:0}@media (max-width:991px){#inbox{max-height:500px!important}}@media (max-width:767px){#dropdown-messages{width:300px!important}.arrow{margin-left:-101px!important}#inbox{max-height:none!important}.mail-conversation-single-message #inbox{max-height:none!important}.mail-conversation-single-message .inbox-wrapper{display:none}.conversation-entry-content pre{max-width:245px;padding:0}}
\ No newline at end of file