When I send mail from an outside domain, it's not coming to my company's domain. When I do send mail from my domain to another domain, it works, but not in Yahoo. I have tested SMTP and DNS configuration by using smtpdiag.exe file and it is all working OK.
I recommend you use a machine outside your company to check your public DNS record. Specifically, have a look at the MX records, and test telnetting to port 25 for each of these records (use the IP addresses associated with each hostname). I suspect that at least one of your MX records is not correct or is missing, and therefore internet SMTP clients can't find your domain.
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