HTML MIMEMultipart issue

[dokime7]

Hi again,

Sorry but I'm trying to increase at maximum the email deliverability :)

I can see on the code than the MIMEMultipart scafolding is well implemented but there is an issue for one case.
The case is when html is provided and no attachment.
In this line we are adding an 'alternative' part into a 'mixed' part but this 'mixed' part is not needed (only when attachments).
So Rspamd add the CTYPE_MIXED_BOGUS flag and decrease deliverability.

[theruziev]

Sorry but I didn't find any spec for changing mimetype from alternative to mixed.

Can you describe more information about it.

[dokime7]

No, it's no changing alternative to mixed, but removing mixed part in case of there is no attachment.
It is not clear on the spec but there is an example with Content-Type: multipart/alternative at the root of email headers.

There is an exemple of scafolding here where attachments are present, but when not, it seems that the mixed part is not needed, so we start directly with alternative part.

We can also read here that the mixed part is used only when attachments on Exchange Server.

Anyway, the purpose of this proposition is to inscrease deliverability by avoiding Rspamd bad score.