Mailbox
I had similar problem last night, was trying to go for the achievement under the ? icon on the HUD. I knew it needed to be mail from players that only counted towards the 10/10. So myself and guildies that were going for the achievement would periodically send each other mail, however I received no more mail once I hit 6/10 until i pitched all my green star system mail (halloween/wintersday promos). Then everything flooded in and achievement unlocked. Not sure if this applies to your situation. Just my two copper on the matter.
Considering all of the requests for support from people who think they didn’t get their items because of this bug, Anet could save themselves some work by fixing it.
The actual bug is, that system messages (the ones with a green star) count towards the mail limit as in preventing you from receiving mails, but the GUI is broken as it tells you, that you would still have space left in your inbox. The achievement actually requires you to have 10 player / gemstore mails and NO SYSTEM MAILs in your inbox.
But don’t expect it to get fixed any time soon. Was broken since early beta, gets reported at least once per week and it still unfixed like so many other bugs, exploits and problems in Bug Wars 2.