How to fix Mailer Daemon?

My boyfriend has been texting me via email for the past 8 months, then all of the sudden he cant text me back because when he does he gets and error that says the Mailer Daemon couldn't deliver the message or something. But I cant still send him texts from my email. Ive tried using 6 different emails like yahoo, hotmail, gmail, ect. And we cant text through aim since aim doesn't support cricket yet apparently. Anyone know how to fix it so he can reply to me?