lotus notes mail address problem

whenever i send a mail to someone my email id gets changed from cthapar@abc.com to c_thapar@abc.com.ihave already tried mailing to local & remote users every where its the same.also the user if someone sends her a new mail locally or through internet it reaches her but if someone tries to reply the mail which she had sent it bounces back as i have mentioned above that the id gets changed when it reaches the user.
also i have tried sending the mail by opening her id on someother machine it the same.
so if someone can tell me how can i get rid of these problem.