When a message is signed, Rebex Secure Mail treats most parts of it (including attachments) as read-only because changing them would invalidate the signature.
The most likely reason for the error you got is that you are trying to send a signed message through an SMTP server that doesn't support "8-bit MIME" or "binary MIME" extensions, but one of the message parts uses "8bit" or "binary" content transfer encoding. In this case, we would normally convert the part to use "base64" or "quoted-printable" encoding before sending.
However, if the message is signed, this is not be possible (it might invalidate the signature) and the proper behavior is to report an error.
If you are signing the emails using Rebex code, a solution for this would be to make sure there are no "8bit" or "binary" MIME parts before signing the message (and change the content transfer encoding to "base64" if there are).
I'm not sure about the webmail, but if it actually skips the check and sends a signed email using "8bit" or "binary" encoding through an SMTP server that doesn't support it, you might end up with an invalid signature (unless the encoding was not actually "8bit" or "binary").
You can force sending of these messages by disabling the check:
smtp.Settings.SkipContentTransferEncodingCheck = true;
smtp is the instance of