When sending mail I get a 'mailbox full error'. I go and look and the character trying to receive the mail has plenty of space in the mailbox. I believe the limit is 50 items and they are 20-25 items under that. Not sure what the problem is. The error seems to come and go. On many of my characters the item icon in the mailbox on the left are messed up. The show strange green aliens, colored boxes and other strange things. Normally this would be a icon showing what item was sent. It doesn't seem to matter what items are sent. The icons are always wrong.
The full mailbox is likely the old bug with ghost mail. There's things in the mailbox that take up slots but aren't visible. The "fix" for that is the /clear_mailbox command, which completely deletes all mail.
Limit is 6 Pages. This includes all messages, not just those with attachments. The in-game mail is not good to use as "storage". Go through your messages and attachments then delete them! If you have read and emptied in-game mail that has attachments, I suggest using /clear_mail. Be warned it will remove all mail on the character you are on when using that command so make sure you have collected all the attachments before using that command.