Created attachment 1359 [details] a short patch to implement a comment line ===Overview=== There is a bug report for KMail mentioning Claws Mail. See: https://bugs.kde.org/show_bug.cgi?id=332036 The Problem described there is most likely a bug in KMail. However, Claws Mail is not totally blameless on that matter. When Claws Mail composes an email containing text and an attached file, the outline of the email will contain something like this: Content-Type: multipart/mixed; boundary="MP_/oLQPONLQse/eBnQ1ahObw=B" --MP_/oLQPONLQse/eBnQ1ahObw=B Although this is correct, most clients insert a comment before the boundary. Like this: Content-Type: multipart/mixed; boundary="MP_/oLQPONLQse/eBnQ1ahObw=B" This is a mime message. --MP_/oLQPONLQse/eBnQ1ahObw=B The comment will not be displayed by a MIME-compatible client, but it seems that at least KMail is expecting it. Unfortunately, KMail takes the matter in its own hands and inserts an extra line where it expects the comment. If the email is signed, this invalidates the signature. The malfuction of other clients may not be the fault of Claws Mail, but the problem can be solved by composing emails with a comment line. Please find a possible way to do so in the attached diff-file. ===Build Date & Platform=== I checked the behavior of Claws Mail with a git clone of today.
Do I understand correctly that Kmail is deliberately modifying contents of incoming messages, and you say that Claws Mail "is not blameless"?
Forgive my ignorance. I have no experience with programming email clients. All I know is that all clients I tested insert a comment line for not MIME-compatible clients (as text or as blank line) except from Claws Mail. So it seems to me, that this comment line should be there. So it is Claws Mail which does not keep in line. However, my concern is not who gets disgraced. My concern is that a problem which I encountered in real life can be circumvent quite easily. Considering the other bug reports for KMail which deal with KMail modifying incoming mail, I have little hope that this will be fixed soon on the part of KMail. So why not keep in line with the habit of other clients and insert a comment line? As far as I see, it would not do harm to Claws Mail and there would be one incompatibility less.