This worked for several years for Microsoft, but has now stopped working - the signature is again shown as an attachment. To get around this, I've historically done this for Microsoft:

The two alternative signatures are presented to the user as attachments, and not inline, so the user only sees download boxes. However, it doesn't work for MS clients (I've only tried ). This displays well in various clients (Thunderbird, and webmail from gmail/Yahoo/AOL/gmx), showing the original message with the appended signature. If the original message was itself a multipart/alternative, then the new message now looks like: I do this by wrapping the original message in a multipart/mixed, and then adding a new multipart/alternative which contains text and html versions of the required signature.

In detail: I need to add a signature to an email message, where the structure of the original message is, in general, unknown. However, insists on putting any additional HTML after the first HTML part in a downloadable attachment, instead of displaying it. Executive summary: I can construct a multipart message which contains HTML in two separate parts, and it displays correctly in multiple MUAs.

Outlook plain text email multipart