We are using an external commercial smtp server for our newsletters (sending them through .NET components), and they offer two smtp URLs - smtp.critsend.com and fast.critsend.com -, and the second one is reserved for sending singular emails, the first one for bulk.
Using nslookup shows that both resolve to the same 4 IP addresses (fast.critsend.com being an Alias).
Question: (how) is it possible for the smtp relay to distinguish between different names? Is there something in the headers that can be compared to host headers in http protocol (I didn't find any intelligible information for a non-sysadmins)?
The reason I'm asking is because we would like to use one of the IPs in our newsletter script (which works) rather than a name (in order to save DNS requests), and we are wondering about potential problems.