"Unable To Send Letter" - Why?

[Pintsized]

Trying to send mail and getting a typically unhelpful error message with no indication at all as to what's wrong!

1) The character is in my friend list, they're even in my FC, the item is a simply Orch. scroll.
2) The character was last logged in 16 hours ago.
3) The character's account is active.

What possible reason could there be for this to fail multiple times? Why do programmers output such totally unhelpful messages when something goes wrong?

[edit]

Turns out the target mailbox is full .. guess SE's programmer was just too damned lazy to bother reporting the fact to me!