Many websites are configured to send email through the host server rather an external (SMTP) service. Typical reasons would be a contact us form, an account activation link or a forum user “watching” a particular forum thread. Since January 2015 1&1 have implemented a new policy that results in non-delivery of email for users that are with 1&1. There are many, many reasons why one of your users isn’t getting email. However, if they are with 1&1 you may well find errors like this in the mail log on your server:

host mx01.gmx.net[212.227.17.4] refused to talk to me: 554-gmx.net (mxgmx110) Nemesis ESMTP Service not available 554-No SMTP service 554-Bad DNS PTR resource record. 554 For explanation visit http://postmaster.gmx.com/en/error-messages?c=rdns

If you follow the generic error message link it suggests you’ve been blacklisted but that is not actually the case as the second part of the message, “Bad DNS PTR resource record” states. Not everyone will know how to fix this so I’m going to show you how. I’ll be using Linode as an example but the principle is true of all hosting. Basically 1&1 are complaining that they don’t like the so-called reverse DNS record set-up for your server. Linode sets a default for you (its usually ends members.linode.com) and its this default that 1&1 finds objectionable.

Login into your Linode controp panel and choose the Linode you want to change.
Click the “Remote access” menu item.
From here you will see a link labelled “Reverse DNS”, click that link.
Enter a hostname in the box provided, e.g. www.example.com or customers.mydomain.com. Now click the Look up button.
You should get a “Match found” message, click Yes underneath the correct address.

Once you’ve done that backed-up mail on your server should start to flow soon after.