After a recent upgrade of our system from ZCP to Kopano (which included upgrading the Gateway server) we noticed that our IMAP Clients will periodically have a problem when sending emails. The email can’t even be saved to Drafts and must be abandoned. But if the user immediately re-composes the email, it sends right away!
The client we are using is Thunderbird (not all are the most recent but many are version 52). There are a number of Knowledgebase articles on the Internet about this, but it seems strange that it would happen only after upgrading the IMAP server.
Is anyone else having this problem? Is there a way to fix it?