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

Hi: I've enabled the checkbox 'Accept cookies from sites' in my 68.11.0 version of Thunderbird as you have suggested but this did NOT eliminate the problems with SENDING messages due to constantly receiving the error 'SERVER timed out error - SEE the attached image ...! I have been trying to resolve this issue for over ONE WEEK but I still CAN NOT RECEIVE MESSAGES - so far NONE of the suggestions/advice I've obtained worked. Due to this VERY URGENT situation - your earlier further help would be greatly appreciated ...! Many thanks. Tony H.

I've enabled the checkbox 'Accept cookies from sites' in my 68.11.0 version of Thunderbird as you have suggested

I don't think I've suggested that. Did you follow the troubleshooting steps in the support article linked above?

Hi christ1: ...and thanks for your reply! In following the troubleshooting steps in the support article linked above - this is what I've tried so far: - checked all SERVER settings and found NO ERRORS - would like to try other PORT 587 (port 465 is my default): would like to send you a screen-shot of the the setting and contact the Email-provider Yahoo Mail to check if they are not blocking the messages - also: do you recommend to try their 'Auto-configuration in Thunderbird' utility ...? - if any of the steps will NOT help - what other step should I try next? I will then let you know about the outcome - thanks for your further help!

Timeouts on sending are often caused by antivirus programs that are set to scan outgoing mail. The best way to test is to run Windows in safe mode, which bypasses startup apps like AV, and then run TB. What is your antivirus, and its mail settings?

https://www.lifewire.com/how-to-start-windows-7-in-safe-mode-2624540