Hi Pragati,
The Sender E-mail address should always be the default e-mail maintained in E-Mail and Fax Settings activity.
Business Configuration work center -> Implementation Projects view -> Select the Project -> Click on Open Activity List button -> Fine-Tune tab ->
Open activity E-Mail and Fax Settings -> Click on E-Mail and Fax Settings link -> check the Default Sender Address (e.g. e-mail address: DoNotReply@myXXX.mail.crm.ondemand.com)
Try to change the Form Address in email channel and check if it is working.
Regards,
Suman