Miss, go into your chat.
If the chat says "You are not allowed to chat!" or something similar you have been gagged.
If it doesn't say that, it means there is an issue with that particular account and I would mail Pet directly as it is a website issue. Make sure when you do that it is very, very clear and you use a subject title that will make it obvious, such as "Mail Issue" or "Mail Glitch". Then you will make a short explanation of what the issue is; don't drag it on, just say it and be done with it.
As for the number of mails in the Default mail, that makes no difference. I have almost 1300 mails in my mailbox and the oldest one is from May 19, 2013. I've never had an issue sending mail under this account as I don't send out large numbers of mails in a short period of time.