I get this error dozens of times a day.

This is really annoying. I would like to know what operation failed.

Try deleting the item from your contacts and reenter it making sure the email address is formatted correctly.

I have 315 contacts. How can I find out which one is defective?

Yeah, that’s a good point.  My suggestion mighht be to delete the account and reenter it.  Since is is a gmail account, IMAP will resync all.

It worked! No error in the last three days. Reentering the account also eliminated the multiple sections in the inbox due to “Important” messages. The inbox has only one thread now. If I get a solution to the “Categories” problem reported in another message, I will be happy with EM Client again.