https://support.mozilla.org/kb/cannot-send-messages

From the new thread (now closed) @ https://support.mozilla.org/en-US/questions/1177650

I followed up the steps sent by christ1, but none of them worked. I think my problem lies elsewhere. Normally, when I first open Thunderbird and attempt to send email, I'm always asked for my password for the SMTP server. I know what it is and, once I typed it in, the mail was sent. That password is different for the POP server password. I don't remember how I configured it in the Account Settings, because I don't see a place for it. Now, for the last 48 hours, the following happens. That password no longer accepted and I'm repeatedly asked to enter a "new password." As I try to re-enter it or try my POP password, the connection to the SMTP server times out. I went back to my SMTP server settings and noticed that my Authentication method was set to "Password, transmitted insecurely." I don't believe that I'd ever set or changed that setting one way or another. However, when I tried to change that setting to "No authentication," my email was sent with no problem. So, could that be the problem? What was Authentication method be set to normally?

From what I had posted above, I think it's clear that I couldn't follow the "Delete SMTP Password" or "Reset SMTP Password" steps. Settings on the Spectrum website provide for changing a POP password, but not an SMTP password.

See my comments in this thread. https://support.mozilla.org/en-US/questions/1177197#answer-1011790

It is not your password it is your account settings per se.