@TastyGM Thanks for the answer!

You may be right, maybe their mailbox is full. I tried sending items to an other person, and it worked. This also explains the occasions when I got the error and even between my alts I couldn't send mail (while currently I can).

It's a shame this cap cannot be changed, it's a pretty annoying feature...

Anyways, thanks again!

I farmed a lot of things in the last month, among them terocone. A person offered me to buy up all the terocone I farm, if I send them in a C.O.D mail to them. I tried doing that in the last 3 days, and the game always said "You have reached the in-game cap of unique mail recipients". I could only mail to my alts.

I read that the reason for this is that I make ~250 auctions a day, and the game considers them among the mail I sent.

This restriction is in place to limit mail-bots offering gold or boosting by spamming people's mailboxes. I don't think that would be a problem on this server, but I wouldn't remove the restriction anyway.

Instead, I think there could be a way for example here, on the website to reset your daily mail quota. It could be protected by a captcha so that botters can't automatically reset theirs.
This way legitimate players doing legitimate business could effectively send an unlimited amount of mail, while it would still be really hard to automate the mailing procedure.