The emails in the sent folder show up on other clients, this tells me that: emails coppied into the sent folder actually do get put there on the comcast email server

So its not like thunderbird is not communicating with the mailserver,

feel free to ask questions and ill poke and diagnose on my end, anything that helps me or helps you to understand this issue is appreciated. Im really looking for how to diagnose and find the logs on my end since im having trouble actually logging the error.

If the message appears in the Sent folder, it means it must have reached the mail server (check the Sent folder in webmail if it's an IMAP account), so what happens after that is beyond the reach of TB. When a recipient claims it 'never actually gets sent', it usually means their mail service blocked it, due to some inadvertent blacklisting of your IP, or it was filtered into a Spam/Junk folder that the recipient may not see if their account is POP. They may also have some security/antispam app on their own system that intercepts mail coming into their mail app.

Interesting to look at. Ive gotten some more diagnostics that make that a little weird though if they send an email from any other mail client it works, Thunderbird on windows or Ubuntu/Pop. and it shows up in sent but actually gets to its destination if they send it from that specific install of Thunderbird on their laptop then it dosent actually get to the destination but still shows up in the sent folder and still appears to send. I think i really should at least get the logs?

Logging is described here: http://kb.mozillazine.org/Session_logging_for_mail/news

Is the exact same message being sent by the other clients? If the one sent by TB has something like a signature, that can be enough to trigger some spam filters at the receiving end.

I think you have to first check the Sent folder in webmail, if the account is IMAP.