any help please ?

Your e-mail server doesn't now that it should process the e-mail from mydomain.com and refuses the e-mail for delivering it, as a relay server. Most servers are refusing relays, because they are mostly used for spammers.  

I'm assuming your server is a linux based system and you are using postfix to receive the email.

Normally you can avoid these errors by doing a good configuration on the used e-mail server (e.g. postfix).  There are a lot of documents (tip: google:  postfix relay errors and use your distribution name and version in the google search).

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