Unable to send mail to <player>

A friend and I have been helping each other out with crafting on our respective jobs so we've been sending a lot of materials and products back and forth using the Moogle Mail service in game. It's convenient because we don't have to be in the same spot or even online at the same time. But lately I've been running into a problem where I'm getting an error message "Unable to send mail to <player>. Items successfully recovered." or something to that extent. My friend claims their mailbox isn't full, so we don't know why this is happening unless there is some sort of "spam filter" in place. But I don't see why there would be considering you have to be friends to send mail.

Any help on this? My character's name is Kenaf Pureblade and my friend's character is Nixie Dangereaux. We play on Goblin.

Do you guys have a third person you could have try sending him a letter? If they send a letter to him and it doesn't go through, that means it's an issue with him and not you.

I believe your friend's mailbox is full. Ask your friend to check mails in the "opened" tab

there may be a message limitation... But im not sure