Having a strange problem here.
We send out emails from a web application that use an SMTP server setup on a Windows 2008 R2 server.
Everything was working fine but one of our customers changed their incoming mail program (that's the best I could get out of their IT department) and now they're rejecting all our emails.
They did some digging and said that they're getting rejected because of non-alphanumeric characters in our originating server name.
Emails originating from our SMTP server come from MACHINE-NAME.DOMAIN_NT.abc. They've told me it's the underscore that is doing it.
Now they asked us if we could change our domain name which of course isn't happening. Another solution would be to relay through another server but that's going to be a pain.
I was wondering, can I spoof the domain name so the originating server will be MACHINE-NAME.DOMAIN.abc or something that doesn't have the underscore?