Thunderbird downloads messages that make into your Inbox on the providers server. If they are not making it in the Inbox ask your provider where your messages are going.

Airmail said

Thunderbird downloads messages that make into your Inbox on the providers server. If they are not making it in the Inbox ask your provider where your messages are going.

Yeah, I asked them for any information (e. g., looking at log files) and they refused to help. If I send from their webmail interface the e-mail goes through so it's not their problem they say (in so many words).

If the messages are not in your inbox it IS their problem. Ask for someone that knows more than to answer the phone when it makes a noise.

Airmail said

If the messages are not in your inbox it IS their problem. Ask for someone that knows more than to answer the phone when it makes a noise.

Yeah I get it but I've tried to get transferred to a more technical person and they refused. I've been through their support a couple times now and they refuse to do more.

Any other ideas on how I can get information? Log files on my PC? Anything?

Are the messages in your Sent file? If so Thunderbird did its part.

Airmail said

Are the messages in your Sent file? If so Thunderbird did its part.

Yes, they are all in the Sent folder. Is there any way to see that the frontier server accepted the e-mail?

We've talked to frontier a couple more times. They are not helpful. I gave them the Message-Id from one sent test e-mail that did not arrive. If it has a Message-Id that means it got to their server correct? I keep asking if they can search for log files by Message-Id. The last call he said he would search for it but I doubt they will. They constantly say they don't support third-party e-mail clients, but they have instructions (that don't work) for Thunderbird on their own website! Any more ideas? They refuse to escalate the issue to someone knowledgeable.

A little more information. If I sending am e-mail to this person via the official frontier web e-mail interface (Yahoo) it goes through. With Thunderbird it saves to my Sent Mail folder but the recipient doesn't receive it. I don't get an error message from Thunderbird, or any bounce back e-mail.

I think I finally fixed it (NO THANKS TO FRONTIER!). It was the outgoing server settings. The settings listed on the Frontier website don't work (of course). But the ones here did: http://madscitech.com/faqs/frontier-email-server-settings-for-yahoo-imap/