Guild Wars 2

Tell him to delete his mails. He probably has 10/10 in his mailbox.

http://wiki.guildwars2.com/wiki/Mail

Seeing how many people is unable to get something as basic as this, or research before creating yet another thread about the same thing in the bug subforum, I understand why Arenanet decided to put the warning about mails before submitting a ticket to support.

Even if his mail is not full, delete old mails. He might need to delete ALL of them.
Also, is his account new? The system may be considering you are doing something illegitimate by sending him something way above his possibilities.

In any case, the new wallet thing has sent a system mail per character (even more if you got sent to an overflow), all of them to add up as you log in with different characters, destroying older mails if necessary. So even if he has space in mail now, if he had 10/10 at the time, he might have lost some of his mails to the wallet warnings… and if the one you were sending him if it was queued.