Here's a problem we've noticed in version 17 (I know you don't call it that, but it's easier than referring to a build #).
When using an email client, like Thunderbird, with IMAP, the messages do not stay in sync. You could have 10 messages in Inbox, then delete 1, and now there are 8 messages in Inbox. Go into webmail, there are 9 messages. Close email client and reopen, back to 9 messages.
There are other sync failures as well, like sometimes the message is read, but it shows up unread in email client. Sometimes you are on one message, but the preview is for another.
These things occur in both the release version of 17 and the current beta. Does not occur in v15.