Hello.

This is a question about the imap protocol, since I am having a hard time finding these things out, so I hope you guys can help me.  We are using Dovecot to receive mails and having an issue with Thunderbird.
Basically, we are running into this bug with a lot of people: https://bugzilla.mozilla.org/show_bug.cgi?id=216308.

The gist of it is, that sometimes, when there is an image linked with the imap:// protocol in the mail, then Thunderbird wants to subscribe to the folder that contains this mail, but fails.
If I pull the mail off the server, it also has that link in the raw mail.  However, most mails store the images as a base64 code block.

I  have actually seen mails getting changed over night, from imap:// links to base64 code blocks, but I can't reproduce that. All computers are supposed to have the same Thunderbird version and they do.

My questions right now are:

* Speaking about the general imap/mail protocol, do mails store images as base64 or imap:// links. What is the standard here?
* Is there anything we can do to prevent Thunderbird from running into this bug? We are using Dovecot 2.2.36.4
* What can cause an email getting changed over night or getting synced back with the base64 blocks? It was already read, when we ran into the issue.

With best regards,

Domeniko Gentner