But I see nothing you can do at your end if you are sending it other than sending a letter to the deferring agent if you have the email, you know who it is. If it is a big company like yahoo or hotmail, I would just send a letter to the recipient and tell them about the issue so they can expect a delay.
It's not a big webmail provider, just a smaller company. I suspect it might have to do with the large attachments, or their mail servers getting overloaded.