When I try to get new email, intermittently I get an error message saying
"Cannot Get Mail - The account ... Is in use on another device."
This is not true - both my other Macs are completely shut down, even after completely exiting Mail.
I am sure nobody else is using my email account.
Just to be sure, change the password to your E-Mail account. After a few days you can change it back if the problem doesn't go away.
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