Have you ever used PGP encryption with Thunderbird? That icon is disabled and has a slash through it when PGP, or E2EE as it is often referred to, is simply not configured for that account.

Thank you Matt for your response and its timeliness! I have never used any encryption tools. FYI, after I posted the issue I went to dinner. When I returned, the message that I wanted to send had been placed in Drafts. I looked at it there and saw the diagnostic message that the email address was not a valid one was correct. I corrected my error (I had taken the email info from a message that I received as the place to respond) and all went well: the message went out with no problem. Thank you again for your response - great service!

Thank you Matt for your response and its timeliness! I have never used any encryption tools. FYI, after I posted the issue I went to dinner. When I returned, the message that I wanted to send had been placed in Drafts. I looked at it there and saw the diagnostic message that the email address was not a valid one was correct. I corrected my error (I had taken the email info from a message that I received as the place to respond) and all went well: the message went out with no problem. Thank you again for your response - great service!

Matt: I just tried to send the message above - got an error message in reply that mention contacting you with what was called a protocol error. Just wanted to help. Thanks again.