Emailing invoices from OE has stopped working

SUGGESTED

Client is on Sage 300 v2018 PU7.  Recently (couple of weeks now) the emailing of OE invoices has stopped working or works intermittently.

It puts up the error message below when it fails:

"Mail delivery failed. Use the Email tab on the Company Profile screen to send a test email and confirm that all settings are correct."

They use Office365.  Nothing has changed so no changes needed in the company profile (I assume).

How can I fix this?

Any and all advice greatly appreciated.

Dana