I recently ran in to a problem with email after moving a website away from my reseller hosting to some new hosting. After the domain name had fully propagated I set up the new email address.
To ensure the new email address was working correctly, I sent some emails from my other email accounts that I hold on Google and GMX. Everything appeared to work correctly, I could send and also reply from the new email address.
I then sent some test emails from other accounts that are hosted on my reseller hosting, which is where the original website was hosted before I moved it.
I sent the email using Rouncube which is a standard webmail client for Linux Hosting. As soon as I sent sent the email, it was immediately returned with the following error message
Mail delivery failed: returning message to sender. No Such User Here.
It left me wondering why this was happening since all the emails I had previously used to send from gmail and gmx worked, but why where they not working from this server?
I later concluded that I had not deleted the old account from the reseller hosting, so obviously when I sent an email using Roundcube on the same server, the email address was still set up and hosted on the old site. After I deleted the old account, I then sent another email and this time it worked.
If you ever run in to this issue, this could be one of the reasons.