This not necessarily a Thunderbird issue. I recall sending such emails to a friend and it took several weeks to discover that he had set his email client to delete incoming messages with attachments. Before confirming the problem, I encourage sending to several email accounts, including your own.

Hi David thank you for your advice but I have tried sending several different emails with attachments on them to different people and none of them have received them these emails show up in the sent box marked as sent. If I send an email on it's own (no attachment) it is received OK? I don't know what else to do.

What happens when you send to yourself?

Hi David I just tried to send an email from myself to myself on my PC and I noticed that it took a while to go and the same problem arose whereby it is showing sent in the sent box but not coming in to my inbox. But upon checking my mobile it has come in to my inbox OK for the same address attachment as well?? Doesn't seem to be a problem there. I have checked my outgoing server and my incoming server but they are reading correct? Totally confused now.

Yes, it's puzzling. If the phone received message and attachment, then the message with attachment was in the server. You could log on to your online account to verify.