TB certainly supports more than 3 gmail accounts, but I suggest you check to see that each account is using the smtp server defined with the same user name/password as the incoming server for that account. Open Preferences/Account Settings, select an account in the left pane, then look at Outgoing Server (SMTP) in the lower right pane. Gmail account 1 should be using gmail smtp server 1, account 2 should be using gmail smtp server 2 etc.

I have gone through the steps that you mentioned. I have made sure that the smtp server associated with my email matches the one that it is supposed to be using. They way I check it is by going thru the outgoing server pane and then looking at each account (attached). The setup and details are uniform across all four accounts and the emails match up to the email account in each case. I just can't use this one to send emails. They are all pop3 accounts. I don't know if that makes any difference.

I notice you have OAuth2 authentication in your first picture. Since OAuth2 is not supported for Gmail POP accounts, I wonder if it matters that the incoming server uses normal password authentication while the outgoing server uses OAuth2.

u've done it my friend! i changed it to normal passwd and it works now. thanks a bunch. what is still a mystery is why the other accounts work fine with OAuth2 but as long as they work, i will leave them alone! thanks again.