diff --git a/README.md b/README.md
index 2d942b1..039806c 100644
--- a/README.md
+++ b/README.md
@@ -264,6 +264,7 @@ export interface SendConfig {
   content?: string;
   mimeContent?: Content[];
   html?: string;
+  relatedAttachments?: Attachment[];
   inReplyTo?: string;
   replyTo?: string;
   references?: string;
@@ -302,6 +303,10 @@ base64 is converted to binary and is only present for a better API. So don't
 encode your binary files as base64, otherwise denomailer can't convert it back
 to binary.
 
+#### relatedAttachments
+
+Attachments related to the html content, for example embedded images.
+
 #### internalTag
 
 This can be used with preprocessors so you can give mail a type, for example
diff --git a/client/basic/client.ts b/client/basic/client.ts
index 5ad73f8..1f4fb70 100644
--- a/client/basic/client.ts
+++ b/client/basic/client.ts
@@ -1,4 +1,6 @@
 import type { ResolvedSendConfig } from "../../config/mail/mod.ts";
+import type { ResolvedContent } from "../../config/mail/content.ts";
+import type { ResolvedAttachment } from "../../config/mail/attachments.ts";
 import { ResolvedClientOptions } from "../../config/client.ts";
 import { SMTPConnection } from "./connection.ts";
 import { QUE } from "./QUE.ts";
@@ -53,6 +55,98 @@ export class SMTPClient {
     return this.#que.idle;
   }
 
+  calcBoundary(content: string, pattern: RegExp): number {
+    let boundary = 100;
+
+    const matches = content.matchAll(pattern);
+    for (const match of matches) {
+      boundary += parseInt(match[1], 10);
+    }
+
+    return boundary;
+  }
+
+  encodeContent(content: ResolvedContent) {
+    this.#connection.writeCmd(
+      "Content-Type: " + content.mimeType,
+    );
+    if (content.transferEncoding) {
+      this.#connection.writeCmd(
+        `Content-Transfer-Encoding: ${content.transferEncoding}\r\n`,
+      );
+    } else {
+      this.#connection.writeCmd("");
+    }
+
+    this.#connection.writeCmd(content.content + "\r\n");
+  }
+
+  encodeAttachment(attachment: ResolvedAttachment) {
+    this.#connection.writeCmd(
+      `Content-Type: ${attachment.contentType}; name=${attachment.filename}`,
+    );
+
+    if (attachment.contentID) {
+      this.#connection.writeCmd(
+        `Content-ID: <${attachment.contentID}>`,
+      );
+    }
+
+    this.#connection.writeCmd(
+      `Content-Disposition: ${attachment.contentDisposition}; filename=${attachment.filename}`,
+    );
+
+    if (attachment.encoding === "base64") {
+      this.#connection.writeCmd(
+        "Content-Transfer-Encoding: base64\r\n",
+      );
+
+      for (
+        let line = 0;
+        line < Math.ceil(attachment.content.length / 75);
+        line++
+      ) {
+        const lineOfBase64 = attachment.content.slice(
+          line * 75,
+          (line + 1) * 75,
+        );
+
+        this.#connection.writeCmd(lineOfBase64);
+      }
+
+      this.#connection.writeCmd("\r\n");
+    } else if (attachment.encoding === "text") {
+      this.#connection.writeCmd(
+        "Content-Transfer-Encoding: quoted-printable\r\n",
+      );
+
+      this.#connection.writeCmd(attachment.content + "\r\n");
+    }
+  }
+
+  encodeRelated(content: ResolvedContent) {
+    const boundaryAddRel = this.calcBoundary(
+      content.content + "\n" +
+        content.relatedAttachments.map((v) => v.content).join("\n"),
+      new RegExp("--related([0-9]+)", "g"),
+    );
+
+    const relatedBoundary = `related${boundaryAddRel}`;
+    this.#connection.writeCmd(
+      `Content-Type: multipart/related; boundary=${relatedBoundary}\r\n; type=${content.mimeType}`,
+    );
+
+    this.#connection.writeCmd(`--${relatedBoundary}`);
+    this.encodeContent(content);
+
+    for (let i = 0; i < content.relatedAttachments.length; i++) {
+      this.#connection.writeCmd(`--${relatedBoundary}`);
+      this.encodeAttachment(content.relatedAttachments[i]);
+    }
+
+    this.#connection.writeCmd(`--${relatedBoundary}--\r\n`);
+  }
+
   async send(config: ResolvedSendConfig) {
     await this.#ready;
     let dataMode = false;
@@ -144,34 +238,10 @@ export class SMTPClient {
 
       this.#connection.writeCmd("MIME-Version: 1.0");
 
-      let boundaryAdditionAtt = 100;
-      // calc msg boundary
-      // TODO: replace this with a match or so.
-      config.mimeContent.map((v) => v.content).join("\n").replace(
-        new RegExp("--attachment([0-9]+)", "g"),
-        (_, numb) => {
-          boundaryAdditionAtt += parseInt(numb, 10);
-
-          return "";
-        },
-      );
-
-      // const dec = new TextDecoder();
-
-      config.attachments.map((v) => {
-        return v.content;
-        // if (v.encoding === "text") return v.content;
-
-        // const arr = new Uint8Array(v.content);
-
-        // return dec.decode(arr);
-      }).join("\n").replace(
+      const boundaryAdditionAtt = this.calcBoundary(
+        config.mimeContent.map((v) => v.content).join("\n") + "\n" +
+          config.attachments.map((v) => v.content).join("\n"),
         new RegExp("--attachment([0-9]+)", "g"),
-        (_, numb) => {
-          boundaryAdditionAtt += parseInt(numb, 10);
-
-          return "";
-        },
       );
 
       const attachmentBoundary = `attachment${boundaryAdditionAtt}`;
@@ -182,19 +252,12 @@ export class SMTPClient {
       );
       this.#connection.writeCmd(`--${attachmentBoundary}`);
 
-      let boundaryAddition = 100;
-      // calc msg boundary
-      // TODO: replace this with a match or so.
-      config.mimeContent.map((v) => v.content).join("\n").replace(
+      const boundaryAdditionMsg = this.calcBoundary(
+        config.mimeContent.map((v) => v.content).join("\n"),
         new RegExp("--message([0-9]+)", "g"),
-        (_, numb) => {
-          boundaryAddition += parseInt(numb, 10);
-
-          return "";
-        },
       );
 
-      const messageBoundary = `message${boundaryAddition}`;
+      const messageBoundary = `message${boundaryAdditionMsg}`;
 
       this.#connection.writeCmd(
         `Content-Type: multipart/alternative; boundary=${messageBoundary}`,
@@ -203,83 +266,18 @@ export class SMTPClient {
 
       for (let i = 0; i < config.mimeContent.length; i++) {
         this.#connection.writeCmd(`--${messageBoundary}`);
-        this.#connection.writeCmd(
-          "Content-Type: " + config.mimeContent[i].mimeType,
-        );
-        if (config.mimeContent[i].transferEncoding) {
-          this.#connection.writeCmd(
-            `Content-Transfer-Encoding: ${
-              config.mimeContent[i].transferEncoding
-            }` + "\r\n",
-          );
+        if (config.mimeContent[i].relatedAttachments.length === 0) {
+          this.encodeContent(config.mimeContent[i]);
         } else {
-          // Send new line
-          this.#connection.writeCmd("");
+          this.encodeRelated(config.mimeContent[i]);
         }
-
-        this.#connection.writeCmd(config.mimeContent[i].content, "\r\n");
       }
 
       this.#connection.writeCmd(`--${messageBoundary}--\r\n`);
 
       for (let i = 0; i < config.attachments.length; i++) {
-        const attachment = config.attachments[i];
-
         this.#connection.writeCmd(`--${attachmentBoundary}`);
-        this.#connection.writeCmd(
-          "Content-Type:",
-          attachment.contentType + ";",
-          "name=" + attachment.filename,
-        );
-
-        if (attachment.contentID) {
-          this.#connection.writeCmd(
-            `Content-ID: <${attachment.contentID}>`,
-          );
-        }
-
-        this.#connection.writeCmd(
-          "Content-Disposition: attachment; filename=" + attachment.filename,
-        );
-
-        if (attachment.encoding === "base64") {
-          this.#connection.writeCmd(
-            "Content-Transfer-Encoding: base64\r\n",
-          );
-
-          for (
-            let line = 0;
-            line < Math.ceil(attachment.content.length / 75);
-            line++
-          ) {
-            const lineOfBase64 = attachment.content.slice(
-              line * 75,
-              (line + 1) * 75,
-            );
-
-            this.#connection.writeCmd(lineOfBase64);
-          }
-
-          // if (
-          //   attachment.content instanceof ArrayBuffer ||
-          //   attachment.content instanceof SharedArrayBuffer
-          // ) {
-          //   await this.#connection.writeCmdBinary(
-          //     new Uint8Array(attachment.content),
-          //   );
-          // } else {
-          //   await this.#connection.writeCmdBinary(attachment.content);
-          // }
-
-          this.#connection.writeCmd("\r\n");
-        } else if (attachment.encoding === "text") {
-          this.#connection.writeCmd(
-            "Content-Transfer-Encoding: quoted-printable",
-            "\r\n",
-          );
-
-          this.#connection.writeCmd(attachment.content, "\r\n");
-        }
+        this.encodeAttachment(config.attachments[i]);
       }
 
       this.#connection.writeCmd(`--${attachmentBoundary}--\r\n`);
diff --git a/config/mail/attachments.ts b/config/mail/attachments.ts
index 95c9ba3..0f488f7 100644
--- a/config/mail/attachments.ts
+++ b/config/mail/attachments.ts
@@ -12,14 +12,16 @@ export type Attachment =
     | base64Attachment
     | arrayBufferLikeAttachment
   )
-  & baseAttachment;
+  & baseAttachment
+  & { contentDisposition?: "attachment" | "inline" };
 
 export type ResolvedAttachment =
   & (
     | textAttachment
     | base64Attachment
   )
-  & baseAttachment;
+  & baseAttachment
+  & { contentDisposition: "attachment" | "inline" };
 
 type textAttachment = { encoding: "text"; content: string };
 type base64Attachment = { encoding: "base64"; content: string };
@@ -35,8 +37,15 @@ export function resolveAttachment(attachment: Attachment): ResolvedAttachment {
       contentType: attachment.contentType,
       encoding: "base64",
       content: base64Encode(attachment.content),
+      contentDisposition: attachment.contentDisposition ?? "attachment",
     };
   } else {
-    return attachment;
+    return {
+      filename: attachment.filename,
+      contentType: attachment.contentType,
+      encoding: attachment.encoding,
+      content: attachment.content,
+      contentDisposition: attachment.contentDisposition ?? "attachment",
+    };
   }
 }
diff --git a/config/mail/content.ts b/config/mail/content.ts
index a83cf47..8d3044e 100644
--- a/config/mail/content.ts
+++ b/config/mail/content.ts
@@ -1,21 +1,47 @@
+import {
+  Attachment,
+  resolveAttachment,
+  ResolvedAttachment,
+} from "./attachments.ts";
 import { quotedPrintableEncode } from "./encoding.ts";
 
 export interface Content {
   mimeType: string;
   content: string;
   transferEncoding?: string;
+  relatedAttachments?: Attachment[];
 }
 
-export function resolveContent({
+export interface ResolvedContent {
+  mimeType: string;
+  content: string;
+  transferEncoding?: string;
+  relatedAttachments: ResolvedAttachment[];
+}
+
+export function resolveContent(content: Content): ResolvedContent {
+  return {
+    mimeType: content.mimeType,
+    content: content.content,
+    transferEncoding: content.transferEncoding,
+    relatedAttachments: content.relatedAttachments
+      ? content.relatedAttachments.map((v) => resolveAttachment(v))
+      : [],
+  };
+}
+
+export function resolveMessage({
   text,
   html,
+  relatedAttachments,
   mimeContent,
 }: {
   text?: string;
   html?: string;
+  relatedAttachments?: Attachment[];
   mimeContent?: Content[];
-}): Content[] {
-  const newContent = [...mimeContent ?? []];
+}): ResolvedContent[] {
+  const newContent = [...mimeContent ?? []].map((v) => resolveContent(v));
 
   if (text === "auto" && html) {
     text = html
@@ -29,15 +55,17 @@ export function resolveContent({
       mimeType: 'text/plain; charset="utf-8"',
       content: quotedPrintableEncode(text),
       transferEncoding: "quoted-printable",
+      relatedAttachments: [],
     });
   }
 
   if (html) {
-    newContent.push({
+    newContent.push(resolveContent({
       mimeType: 'text/html; charset="utf-8"',
       content: quotedPrintableEncode(html),
       transferEncoding: "quoted-printable",
-    });
+      relatedAttachments,
+    }));
   }
 
   return newContent;
diff --git a/config/mail/mod.ts b/config/mail/mod.ts
index 80ad9b5..a050ca5 100644
--- a/config/mail/mod.ts
+++ b/config/mail/mod.ts
@@ -3,7 +3,7 @@ import {
   resolveAttachment,
   ResolvedAttachment,
 } from "./attachments.ts";
-import { Content, resolveContent } from "./content.ts";
+import { Content, ResolvedContent, resolveMessage } from "./content.ts";
 import {
   isSingleMail,
   mailList,
@@ -28,6 +28,7 @@ export interface SendConfig {
   content?: string;
   mimeContent?: Content[];
   html?: string;
+  relatedAttachments?: Attachment[];
   inReplyTo?: string;
   replyTo?: string;
   references?: string;
@@ -48,7 +49,7 @@ export interface ResolvedSendConfig {
   from: saveMailObject;
   date: string;
   subject: string;
-  mimeContent: Content[];
+  mimeContent: ResolvedContent[];
   inReplyTo?: string;
   replyTo?: saveMailObject;
   references?: string;
@@ -69,6 +70,7 @@ export function resolveSendConfig(config: SendConfig): ResolvedSendConfig {
     content,
     mimeContent,
     html,
+    relatedAttachments,
     inReplyTo,
     replyTo,
     references,
@@ -84,9 +86,10 @@ export function resolveSendConfig(config: SendConfig): ResolvedSendConfig {
     bcc: parseMailList(bcc),
     from: parseSingleEmail(from),
     date,
-    mimeContent: resolveContent({
+    mimeContent: resolveMessage({
       mimeContent,
       html,
+      relatedAttachments,
       text: content,
     }),
     replyTo: replyTo ? parseSingleEmail(replyTo) : undefined,