Thoughts off the top of my head:

• time-outs (mentioned above)
• memory/space issues (mentioned above)
• program code issues(mentioned above
• unexpected data

The first three were mentioned above, and in better detail than I could do.

What do I mean by that last one? The possibility of invalid data (invalid character in or bad/unexpected email address/name/whatever). You might consider wrapping your sending code/call in an eval() statement then examining the values of the $!, $@, $?, and/or other variables (as appropriate to your code) and logging any content they have (along with the data you used on that pass).

I hope that helps, and hope what monks wiser than I will chime in, so we both may learn.