@ghostlands I suspect these may be sync messages though given your “steps to reproduce the problem” i can’t be sure. Please check Encrypted attachments in regular email - #19 by iequidoo also, maybe it will be helpful.
EDIT: Now Delta Chat syncs many things, not only contacts. E.g. some settings, chats archived/muted states etc.
EDIT:
It would be very bad to trend in a direction where Delta Chat can’t be used in conjunction with other email clients without complication, awkwardness, or danger.
No, there are no plans to complicate using Delta Chat in conjunction with other MUAs. All such problems should be fixed.