Take the 2-minute tour ×
Server Fault is a question and answer site for professional system and network administrators. It's 100% free, no registration required.

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.

share|improve this question
    
"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)" TTL? DNS Cache? What if the provider changes IPs? What if the IP you choose goes down? (They most likely have 4 for DNS round robin/failover). If They gave you a hostname, use a hostname. –  sam Nov 16 '11 at 10:34
    
We do read them in advance. It's just that for some reason (which we are still trying to find out) the DNS doesn't always answer. So, if the script lookup works, it's fine, and if not, we use the IP. Sorry, I should have elaborated. –  Olaf Nov 16 '11 at 10:37
    
OK that kind-of makes sense, but if the DNS doesn't respond you probably have an issue somewhere else. Oh well, each to their own –  sam Nov 16 '11 at 10:39
    
True, there IS an issue. We believe that the hosting provider DNS blocks requests from our server's IP if there have been to many in a short period of time. But that's just what we have to deal with, and handling that stuff with a hotline - which we keep trying - can be arduous. So we are looking for workarounds that are relatively reliable. This is real, not ideal, world stuff and, being developers, not sysadmins, we are certainly not happy about needing to dig into this. So please bear with us. –  Olaf Nov 16 '11 at 10:49

1 Answer 1

up vote 0 down vote accepted

The 4 ip addresses you are seeing using nslookup could just be gateway servers for the network using round robin dns for fail over / load balancing.

These gateways will relay to different "back ends", the only way to really be sure if this is occurring is to send test emails to yourself and inspect the email headers (unless they do a good job of purging internal ones), and see the different servers being use to send the emails.

share|improve this answer
    
Thanks, understood, so far. We'll run the test. Yet the question remains: how is SMTP working in that regard, compared to HTTP? Are there names in the headers somewhere? –  Olaf Nov 16 '11 at 10:40

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.