If client supplies message size with no max size configured it is rejected.

[laeubi]

With no message size set (default = 0) every message is rejected if message size is passed.

in MailCommand:
// Reject the message if the size supplied by the client
// is larger than what we advertised in EHLO answer.
if (size > sess.getServer().getMaxMessageSize())
{
sess.sendResponse("552 5.3.4 Message size exceeds fixed limit");
return;
}

there should be a check (like on the EhloCommand) that the MaxMessageSize is larger than 0