Chatmail accounts should not use "move-to-deltachat"

[hpk42]

Delta Chat should detect XCHATMAIL capability when connecting to IMAP and disable "move-to-deltachat" in this case.
Whether the server is a chatmail server should also be available to the UI via is_chatmail config value. The value of is_chatmail should be reset on reconfiguration and set when connecting to IMAP so it is set even for existing chatmail accounts.

Corresponding chatmail PR: deltachat/chatmail#278

[hpk42]

i think XCHATMAIL is better as a marker.

[adbenitez]

I think all that is needed actually is just to disable the permissions to create folders, some servers already do this I think and Delta Chat tries to create the DeltaChat folder there, fails and then since there is no DeltaChat folder it only watch INBOX,

a XCHATMAIL flag might still be useful for other purposes but it doesn't avoid ex. old clients still creating folders

[hpk42]

looked a bit in dovecot docs but did not find a config to prevent folder creation. I guess one needs to play with unix-filesystem permissions to do so. An XCHATMAIL marker can also help to tweak other behaviours -- e.g. #5381