[ancientsofmumu]

I'm a (paying) Mailfence customer as well, and use it as a destination sync backup of my primary domain using imapsync, encountered a similar problem so opened a ticket to Support.

The general problem is that Mailfence has what I guess you could call DDoS protections in place and opening too many simultaneous IMAP threads trips their defenses. I have my ~/.mbsyncrc set to only use one thread when writing to Mailfence to avoid tripping the trigger and it runs like clockwork now (cron timer doing IMAP sync writes from the primary). $0.02, hope this helps.

[tut-urut-utut]

Thanks for the confirmation. I supposed it was some kind of rate limiting. I guess Claws worked for me since its IMAP operations are single threaded, and with Thunderbird, it was easy to trigger multiple parallel connections, e.g., by opening an IMAP folder while copying of messages is running.