RE: Any idea what the problem is. Yes - sending an attachment larger than 2mb.
You can check with your hosting provider but it sounds like that is their limit.
But why would you want to do it. Sending email attachments larger than 2 Mb is always likely to be problematic. Lots of users won't want to receive attachments larger than that. I certainly don't want people emailing me 20 Mb attachments without asking me first.
Would be more common to include a download link in the email for anything approaching that size.