If you have multiple smtp servers in Tools/Account Settings, in the lower left pane, Outgoing Server (SMTP), then you can specify which smtp is used by a particular account by selecting the account in the left pane of Account Settings, then looking at the Outgoing Server (SMTP) drop-down box in the lower right pane. Up to now, it appears you had all accounts sending through smtp.isp.com on port 26. Can you send from your new account through the same smtp? That depends on the account provider's policy for sending over a non-provider smtp. Since port 26 is as (un)secure as port 25, you can probably do it - try.

But it's generally not recommended to use unsecured smtp servers anyway, since they seldom work with mobile devices. In cases where the ISP or mail provider doesn't offer secure connections, it's usually recommended to add a gmail or equivalent secured-connections account to the device and set the other accounts to send over the secure gmail smtp. Adding the accounts to the gmail account as 'Send mail as' accounts in gmail Settings makes the gmail account practically invisible as far as recipients are concerned.

This helped me to get going in the right direction. I went back to the ISP and they helped me set up a webmail thru them. I entered the ISP's info into the outgoing server part of Thunderbird Settings and out went the emails!

While I found lot's of info on the blocked port 25 problem, I didn't find as much clear answers on how to fix or get around the problem.

Thanks!

I'm glad you've got it working, but the short answer to the port 25 problem is 'don't use it'. It's a leftover from the early days when most email was sent from desktop computers using a single POP account that was provided by the same company that provided the internet connection for the desktop. It still works in those limited situations, but is not relevant to the current world of mobile devices, IMAP mail and secure authentication.