From protocol logs and other information it looks like a lot of problems in eM Client v8 started when they decided to handle a special case of an IMAP + SMTP server co-operating to deal with sent items.
Some server automatically make a copy of mail submitted by SMTP into the IMAP sent folder. So eM Client tries to check for this before copying to sent.
The way it checks is to do a SEARCH on the sent folder for the Message-Id field in the email. This is the problem. Message-Id is inside the email, and to find it requires reading the email. Also sent folder can be extremely large. So a search for this can take a long time if the Message-Id field isn’t extracted when the message is appended and indexed.
What’s worse, it does this for every message. It’s supposedly supposed to time out, after about a minute and copy to sent, but clearly when it times out the search, it does not copy to sent, and gets in a knot.
So, if you have any option on your mail server to index Message-Id in a folder, maybe turn it on, and the eM Client won’t time out this search, and everything may run smoother.
Hopefully the guys at eM Client will abandon this idea of this search. Or at least give us an option to disable it. It seems to affect many core functions of EMC, like even reading mail.