Symptom
I suddenly receive 1000++ mail from email, subject is Mail Delivery failed: returning message to sender

Reason behind
There are Virus / Trojan in the infected computer and keep spamming mail to others. And it over the limit, so hosting return all other mail.

There is a maximum outgoing email limit from shared hosting
Maximum Send Mail (per hour)        300        
Maximum Send Mail (per day)        1000        

Resolutions
1. Change email password from webmail immediately.
2. Identify all the computers which have been used to log into this email account, and perform a malware scanning using an ANTI MALWARE software with the latest definition updates.
3. DO NOT save the new password in Email Application at the moments, until the problem solved.
4. If the problems arises again, repeat Step 1 and request support from us.