Cannot send mail

I don't think I've seen this type of issue reported before.

Go to Sources in the mailbox list and check that “Mailbox > Mailbox Type” makes sense for the Drafts and “Sent Messages” types (and maybe also the other available categories).

What happens if you just save a draft without sending?

Is it a new issue, that is, has it ever worked for you?

Yes it worked before. The situation is really strange: when I try
to send a messgae, I get a new entry in Drafts. When I go to the
source of that message it shows the Archive folder in my imap
mailbox. However the message is not in the Archive folder (I have
a script on the imap server that processes the Archive mailbox).
Even after synchronizing, mailmate insists that the mail is sill
there, but it is definately not.

When I compose a message and I abort it, and choose Save when the
popup asks me if I want to save a Draft, it is not shown in the
Drafts folder. (But it does get save to the Archive folder)

Sending mail from another client (e.g. my mobile phone) works as
expected.

The mailbox type for Archive is Archive, and that for Drafts is Drafts.
Also the others are as expected.

It definitely sounds like the Archive mailbox has a “Drafts” mailbox type, but that wouldn't even explain why the message is not sent (except if saving it to a different folder means that, e.g., the draft-state of the message is removed by the server).

Let's focus on 1 problem: Opening the composer and hitting “Save” does not save the email to the Drafts mailbox of the corresponding account under Sources in the mailbox list.

What about the Drafts mailbox in the “Mailboxes” section of the mailbox list? Does it contain the email after saving?

For me to possibly reproduce the issue, use the address seen when using “Help > Send Feedback” to send me the following files:

/Users/<username>/Library/Application Support/MailMate/Sources.plist
/Users/<username>/Library/Application Support/MailMate/Submission.plist
/Users/<username>/Library/Application Support/MailMate/Identities.plist

A saved composed message is not in the drafts box on the sever, nor in the Mailboxes section.
A sent message ends up in the Drafts folder in the Mailboxes section, not in the one on the server.
The files are sent via feedback

I confirm this fixed the issue. No idea how the plist file could have been changed. I never did that for sure.
Thanks!