Mail server settings - Incoming issues

Mail server settings - Incoming issues

Hello, 
We have configured Mail server settings—Incoming, but we are getting the following notifications:
It says the following: “The emails were not retrieved due to an unusual connection with the mailbox. Check the settings and try the operation again.”

We have verified the Mail server, and we get that it is working properly, yet we have got twice the message. 
Could you assist us with the issue?

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