mails are bouncing even the mail address is correct

mails are bouncing even the mail address is correct

i am copying below the msg i receiving from other receiver back as bounced even when its a regular and active email address. everytime we are facing these kind of errors. kindly rectify it on immediate basis. thanks.

This message was created automatically by mail delivery system.

THIS IS A WARNING MESSAGE ONLY.

ERROR_CODE :550, ERROR_CODE :Invalid Sender [502]

YOU DO NOT NEED TO RESEND YOUR MESSAGE.

The original message was received at Sat, 09 Sep 2017 05:42:40 -0700
from accounts@alliancegulf.com [ accounts@alliancegulf.com]

----- The following addresses had fatal errors -----
[Status: DOMAIN_NOT_RESOLVED, Address: < syed@alliancegulf.com>, DOMAIN_NOT_RESOLVED, Status: DOMAIN_NOT_RESOLVED, Address: < wasti@alliancegulf.com>, DOMAIN_NOT_RESOLVED]