Change the server to smtp.gmail.com (and the incoming to imap.gmail.com or pop.gmail.com), remove the oauth:// entry for the account from Saved Passwords in Settings, restart TB, and enter the account password in the OAuth window, then see if sending succeeds.

Thanks! That worked!

I was able to send a couple of emails out but now I'm experiencing the same problem. Do you have any other ideas?

Do you use a VPN? What changed between then and now? There have been gmail outages in several locations in the last day.

I don't use a VPN but I use Microsoft 365 through a volunteer organization. I checked with others and they're not having any difficulties. I don't know anything that changed. I've had no difficulties receiving emails today. This may help... In looking at my password file, I notice that 2 of my 3 gmail accounts have imap and smtp listed but the other one (my main one) doesn't have either. All 3 have oauth. I'm assuming that all three need an imap and an smtp. Correct? If so, how can I add them? Thanks

P.S. When I go under Account Settings, Server Settings all 3 gmail accounts show the server name as imap.gmail.com

I have been able to send test emails between my 3 accounts.

There should be a single oauth:// entry for each account that works for both incoming and outgoing. Remove the imap:// and smtp:// entries for those accounts.

Thanks! I think that solved the problem. One more question... I notice that sending messages seems a lot slower lately. Do you know of any reason for that?

I appreciate your prompt responses!

Sending speed can be degraded if you have an antivirus scanning outgoing mail, e.g. Avast Mail Shield. The general recommendation is to exclude the TB profile folder from AV scanning and to disable scanning of secure (SSL) connections, leaving the real-time background scanner in place.

http://kb.mozillazine.org/Antivirus_software

I don't think that's the problem, but I will read the link more carefully. Thanks.

Can the volume of emails in the inbox, or in folders cause outgoing mail, to be slower?

Ideally, you want to keep the number of messages as small as possible, especially in Inbox, but it probably takes tens of thousands before performance is affected, barring other factors like AV, network performance etc. The All Mail folder can get quite large, but most users unsubscribe it since it's redundant.

If you run Windows in safe mode, that will test the effect of startup apps like AV, since they are bypassed in safe mode.

That is very helpful to know. My All Mail folder has almost 25K messages. On more than one occasion I have found this folder very helpful when something goes missing for some reason, so I'm hesitant to unsubscribe to it.

There is also another folder called "Important". It's also quite large. I assume these are also included in the "All Mail" folder? If so, perhaps I could unsubscribe to that folder to cut down the number of emails.

What are the criteria to make it go into "Important"? Is there any reason why I shouldn't unsubscribe to this folder instead of the All Mail folder?

In gmail webmail settings, Inbox, Importance markers, it's stated"

"Gmail analyzes your new incoming messages to predict what's important, considering things like how you've treated similar messages in the past, how directly the message is addressed to you, and many other factors."

All received and sent messages are in All Mail, so I would think that includes the Important folder, unless you choose 'No markers'. In my setup, only Inbox, Sent Mail, Spam and Trash are subscribed. If I need to access All Mail, I do so in webmail.

http://kb.mozillazine.org/Gmail

Thanks! It looks like I have some reading to do.

I appreciate your information and prompt responses!