ISA mailbox full, email rejected.

Hello,

I am starting to gather documents for applying for a CoE, and since I have some questions, I emailed the Immigration Service Agency to info-tokyo@i.moj.go.jp , but after a few minutes I got an automated reply saying that the email box is full, and to try again later or contact them directly.

Is this common? In your experience, should I try a few days later, or try calling them by phone?

Thanks,