SMTP Reverse DNS Mismatch

Question

My dedicated server ns382087.ovh.net resolves to 46.105.98.72 and hosts multiple domain with several dedicated IP addresses. Each new IP address is added to /etc/network/interfaces

Among them, the dedicated server hosts my domain grand-manitou.com which resolves to 46.105.48.41

MXToolbox.com states that there is an issue when performing SMTP Test: Warning - Reverse DNS does not match SMTP Banner

My question is: how can I solve this problem?

Edit: Is there a workaround to configure my domain grand-manitou.com so that it designates ns382087.ovh.net as smtp/mail server ?

Solution 1

When your mail server identifies itself at SMTP time, it says its name is:

220 ns382087.ovh.net ESMTP

However, the SMTP server (obtained from grand-manitou.com's MX record) is actually mail.grand-manitou.com at 46.105.48.41.

The solution is to configure your mail server to identify itself correctly as mail.grand-manitou.com.

Solution 2

I go another way. FOrget your domain, that has no part.

THe server name is ns382087.ovh.net - that is what the SMTP identifies itself as. Irrelevant what the domain is named that has the emails. OTherwise providers could not use one server for many domains (only one PTR ever exists).

The problem is that a PTR lookup on ns382087.ovh.net must include the IP of the server, and ns382087.ovh.net must go to the same IP. That way someone can see that yes, the server is the server that should be there.

Again, your domain is totally not part of that - this server check happens before the user email domain is checked. Just make sure your server name map forward and packward to the IP and that the SMTP agent uses the server name.

It is quite common for anyone outside smallest stuff that this is NOT the domain name - domains may be served by multiple servers. Or you may use a specific token name here (smtp.yourdomain...) All that is important is that the name of the SMTP service during handshake relates to the DNS in the proper way.

Check DNS and PTR for SMTP: shared IPs and subdomains for another explanation of things.

Solution 3

$ host -t mx grand-manitou.com
grand-manitou.com mail is handled by 10 mail.grand-manitou.com.

$ host mail.grand-manitou.com
mail.grand-manitou.com has address 46.105.48.41

$ host 46.105.48.41
41.48.105.46.in-addr.arpa domain name pointer grand-manitou.com.

grand-manitou.com != mail.grand-manitou.com

Your ovh server doesn't enter into it and will never receive mail for grand-manitou.com

Solution 4

your MTA helo's with ns382087.ovh.net instead of grand-manitou.com. some spamfilters compare the SMTP helo to the (forward confirmed) reverse dns of your mailserver, so it's usually best to have HELO/rDNS/A-record in sync.

set the SMTP HELO to "grand-manitou.com". this can usually either be done by changing the system hostname (many MTAs get the default helo from that) or by overriding it in your MTA's configuration.

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Comments

[Ladadadada]

Sending email from OVH is a fool's game. They're only just outside the top 10 worst spamming ISPs according to SpamHaus and they're often in the worst 10. You might notice that some of those issues are blocking entire /24 ranges and have been blocked for nearly a whole year. Your IP is already blocked by three DNSBLs.

[Gryphius]

while you're technically correct, leaving HELO/rDNS to a generic nsxxx.ovh.net name will give you additional spampoints or even rejects on some spamfilter systems. it's usually better to configure a hostname under your own control so the receiving side knows this vps was meant a legit mailserver and isn't just yet another owned botnet drone.

[nickgrim]

MX records are for incoming mail, and are only tangentially related to the servers that send outgoing mail - they're often the same, but they certainly don't have to be.

[adaptr]

This is rarely if ever an issue; he needs FCrDNS, not EHLO.

[Gryphius]

dig -x 46.105.48.41 +short yields grand-manitou.com , dig grand-manitou.com +short yields 46.105.48.41 where do you see a fcrdns problem?

[TheFoOL]

I should have added that multiple IP addresses are rerouted by my ISP to my server. auto eth0:1 iface eth0:1 inet static address 46.105.48.41 netmask 255.255.255.255 broadcast 46.105.48.41

[adaptr]

Then this - and only this - is the reason most MTAs will refuse you. You should bind your mail server to the correct outgoing address.