It’s not uncommon for Windows web servers to use the IIS SMTP Server as a mail relay. It’s a smart host in that it can do the MX record lookups to find the mail server handling the recipients’ mailboxes. But there is a problem. The big mail services such as Google and Microsoft are very picky about which mail servers they will communicate with as they try to mitigate the never-ending stream of spam. To this end, they will generally only accept mail from servers that speak ESMTP (Extended Simple Mail Transport Protocol) as currently defined in RFC 5321. Use of ESMTP is considered optional, with a fallback to the older SMTP commands if one mail server doesn’t support ESMTP. But because spammers are now the primary users of older SMTP, the big mail services simply drop the connection if the sending mail server doesn’t use ESMTP.