I am getting following error while sending email to Emailservice

(Undelivered): 554 common.apex.runtime.ParseException  What would be the problem.

The code was running perfectly till last Friday 21-Mar-2015. I haven't change anything in code since then, but it's not working since two days; stange behaviour. Although even If I had any problem in my code, email should receive in Salesforce and then I can use debug log to see any code exception but I am receiving bounce back email with above error message.