Sender’s mail ID improper

While sending out mails, the recepient gets an improper SMTP ID. Hence any replies to the ID does not get delivered. But if the correct ID is retyped the message gets delivered.
We have checked for the fields of SMTP ID that are reflected at the recepient end both in server and client.
Another observation is that such corruptions happen when sent to Yahoo mail ID but not to hotmail. ( Hotmail users receive properly)
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