Did you try this one?

https://addons.mozilla.org/en-US/thunderbird/addon/lookout-1/

Thanks for the suggestion. However, it claims its not compatible with TBird ver 56.

Try it anyway, as many add-ons work in newer versions despite the 'disclaimer'. Drag the Download Now button and drop it onto Tools/Add-ons in TB to install.

The latest general release for Thunderbird is 52.4. Any version higher that that would be beta. Do not run beta software if you want compatible add on.

Sorry! My version is the latest 52.4 (must have been confused with FF) While I was able to get this installed I have yet to get it to work with the target exchange server yet. Once that happens I'll update accordingly. Thanks for the suggestion(s)

ISTR that Lookout is used to decipher TNEF attachments sent out by Outlook/Exchange users. I didn't think it would help in making a connection to the server.

Ideally, the server's operator will enable IMAP or POP and then it works like any regular email server. If they refuse to do this, you will need an add-on to mimic the protocols that Microsoft email systems use. There aren't many of these add-ons. :-(

https://exquilla.zendesk.com/hc/en-us

Airmail said

The latest general release for Thunderbird is 52.4. Any version higher that that would be beta. Do not run beta software if you want compatible add on.

OTOH, some authors provide beta versions of their addon. Often under developer comments or on the support website. (but apparently not in this case)