Could not connect to mail server xxx.com; The Connection was refused

[Sea Walker]

Using Thunderbird mail client on win 10 machines. Same internal network segment as NAS running MailPlus Server. Cannot retrieve mail using IMAP. Thunderbird client responds with:
Could not connect to mail server xxx.com; The Connection was refused

Can anyone offer any assistance. I am NOT new to mail servers, installed and ran GroupWise mail servers for many years.
Thank you!
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