Is each gmail account sending on the smtp server associated with each account? Gmail requires the outgoing server be the same (user name, password) properties as the incoming server. Open Tools/Account Settings, select a gmail account in the left pane, then check Outgoing Server (SMTP) in the right pane. You may need to edit or create separate smtp servers for each gmail account, and give them unique Descriptions.

Thanks for the reply. I'm not sure I totally understand -

All the Gmail accounts (2 working and 1 not) were setup by Thunderbird and have smtp.gmail.com for the outgoing server and all the parameters I've looked at in all 3 accounts are the same except the User name and email address.

I noticed an additional related issue - when I click on get messages, a message appears in the lower right hand corner of the screen saying "The current operation on "Inbox" did not succeed. The mail server for account (the one that doesn't work) responded: Unknown command .......)"

It appears you have 3 gmail smtp servers, each with a distinct user name and password, but you must check that the non-working account is sending on its smtp server, not any other one. Is it a POP account? That would explain why you only see the Inbox; the Sent folder will appear once you are able to send a message.

There have been several Gmail issues recently, most of which are supposed to be corrected in the latest TB version 52.2.1.

Something really squirrel y is going on. I first checked in Gmail if the account was a POP acct. I found it was indicated for both POP and IMAP so I unchecked POP and the account was able to send. So, I did that for all the Gmail accts. then all but a different one could send. It seemed I'd fix one and another would quit working.

I then insured they all were pointing to a different Outgoing Server as you said. That also would work for a while then another random account would quit. I then changed them all to use the Default Outgoing Server and they all are now able to send messages. However, two of them still do not have any folders but Inbox and they both receive the "Inbox error" above when I Get Messages.

I am on TB 52.2.1

Any more ideas?

Thanks again!

I think you should decide if you want the accounts as POP or IMAP, move the existing messages to Local Folders for backup, remove the accounts, read the knowledge base article that explains all the password and security issues that need to be considered, then add the accounts to TB. Before re-adding the accounts, make sure there are no leftover passwords in Tools/Options/Security/Passwords/Saved Passwords.