I still didn’t set up chatmail server, but another forum user did.
He set it with help of chatmail/relay/cmdeploy script.
After the setup was done, he comment out imap_capability = +XDELTAPUSH XCHATMAIL in /etc/dovecot/dovecot.conf
After Dovecot restart, DC-specific capabilities were no more listed in IMAP server responce at least before the succesfull login.
Server and clients are doing ok.
Even mobile notifications seems like working for now (but see this
How to install own chatmail and notification proxy? - #12 by link2xt )
What are we missing here?
If DC-specific capabilities info is actually used by DC core, imho it should be accuired after the login. For IMAP server to change (update) server capabilities answer after succesful client login is perfectly possible by RFC.
But I didn’t find a way for stock Dovecot to sent different capabilities to non-authentificated and authentificated users.
The ‘fastest’ way in network terms is probably to send ‘real’ capabilities with ‘OK’ server answer on successful login (with post-login script probably); but it’s not clear for me yet is it allowed by RFC or not.
Client (core) may also send CAPABILITIES command after the login. Maybe it can be answered by a script, so patching the Dovecot will be not needed. Will look into it later.