I’m chiming in, to clarify a bit more…
In the Mailcow footer section i had setup both types with the same content: the PLAIN version (only text) and the HTML version (witch only used some text formatting like bold & italic). I guessed, that for every format of an eMail i want to have a corresponding footer.
The problem here is 100% reproducable:
1) create new email and write some text
2) make a screenshot and paste that after the text (now we’ve a inline graphic)
3) write some more text (after the pasted graphic)
4) add attachements (we had PDFs, not big at all 60 KB)
5) send eMail
Now on the receiver side the eMail shows no attachements, but everything else is displayed correctly (text + inline graphic + text). Even SOGo does not show the attachements.
BUT: almost all eMails (only very few didn’t) contain the attachements (looking at the raw source of the email, the multi-part segments are there, which also have the correct file names of the attachements).
Crosscheck
On the sender side i use the (before) sent same eMail (with inline graphic and attachments) and:
1) delete that inline graphic, keep the attachements and send again (to the same adress): success!
2) or leave it as it was (text, inline graphic, ataachements), but only disable the mailcow footer, send it – success!
So my thought is, whenever the automatic footer is active, somehow mailcow (or probably rspam) does somewhere an encoding mistake (not properly positioned or closed tages, etc.) and then the interpreter on the receiving side simply falls over it and strips the rest (which is then the attachements).
Else i can’t explain it – removing the inline graphic, or disabling the footer results in 100% success.