Undeliverd Email

I use Mozilla Thunderbird and Comcast IMAP server for my Xfinity emails. Some outgoing emails that are 'replies' and have the original messages quoted in-line are not being delivered to the recipient. This is happening only on one of my comcast email accounts, also the one I use the most. The 'sent' folder shows the message as sent. Specifically if there is an attachment in the original email, then my reply does not reach the recipient. I have been talking to comcast for a while now with no resolution in sight. Any ideas??