Apple Mail issues

I use the Apple Mail client on my iMac and iOS devices. I have icloud e-mail addresses. Generally it all works smoothly but every few weeks it follows this pattern:
(1) sending mail slows to a crawl. Messages stay in the Outbox for seconds, even minutes
(2) a new folder/mailbox appears called “Recovered mail” or something like that. This contains the sent message that had been delayed in the Outbox.
Does anyone else see this?
I delete the new mailbox almost immediately.

I’ve not experienced this. Have you tried using a dummy e-mail address to determine if it’s a server side issue or if the issue only persist in the client itself.

Thanks for the suggestion. Problem is that the issue is intermittent. It can occur on one message but not the next a few seconds later.

Have a look at this article. Since you said you are seeing the problem on your iMac, that may be where the problem is.

Thank you. Unfortunately that article is from 2012 and the current version of Mail doesn’t have a .offlinecache file. However, when I examine my Mail folders in Finder I see dozens of .emix files in the Deleted and Draft folders for messages that do not appear in I think I need to rebuild my entire Mail system from scratch.