How is this related to Thunderbird? Did you have the recipient check the spam folder?

The emails are not in spam. It is random. Most go thru, but a few get lost.

Once the message appears in your Sent folder, it really has been sent. From that point on there's nothing you or Thunderbird can do about message delivery to the recipient. You may contact your email provider for help and have the recipient doing the same.

I am having the same problem.

I have two Gmail accounts, when I send an email form one to the other via Thunderbird it is not showing as being received in my second Gmail account.

I also sent emails to people over a week ago and never got a reply, however I tried again but this time I sent the same emails to the same person using Gmail interface and got a reply within the hour.

Any I suggestions please.

As stated above, once your email providers SMTP server has accepted the message Thunderbird has done its job. Contact your provider and ask them where your mail is going.

I am sending "test" messages to myself via cc: using ix.netcom.com and they don't even show up in Webmail. The "To" recipient is not getting the message and it is not hung up in spam. I send to these people all the time. The problem just started happening.