A 550 mailbox unavailable error should be specific to one recipient email address, usually the result of an invalid address.  It should also generate a failed delivery bounce back message that may provide additional information.

I have two computers (A & B) in a home environment. Both are running Windows 7.

Our internet provider is Verizon. When Verizon dropped servicing email, we began service by AOL, using our prior verizon email addresses (user@verizon.net)

We do not use the cloud option for creating and reading e-mails.

We use Pegasus.4.70 build 546 on both computers. I have checked the setup parameters, and I see no differences between them.

No problems on either system for over a year from the cut-over to AOL.

At some point, with no changes on our end, when we send a message on System B, we get the dreaded ERR 500 message (Server not available)

AOL says too bad for us, since the web can send messages, and they can’t help with the client.

Is there somethong I should look at more carefully?

Enable internet session logging, attempt a send, then review the log for additional clues.  Don't post the entire contents of a log as it contains credentials, possibly encrypted but it's easily decrypted.  Whatever is causing the problem will be at the end of the log.  Don't forget to turn session logging off when done troubleshooting.  Log files are located in a \TCPLogs subdirectory of the Home Mailbox directory.

Some mail servers will reject message where the From: address doesn't match the credential email address.   This would be the "My internet address is" setting located in the General tab of Internet Options.

Try creating a new SMTP host.  Sometimes things look correct but something unseen causes failures.  A new configuration file will rule that out.

Brian,

Thanks for the quick reply.

Sorry, I had a mistake in the message.

The last line in the log is ERR 550 mailbox unavailable.

Dennis