Is "jotdot@somedomain.com." actually invalid? My understanding is that something...

robomartin · on Jan 3, 2013

I think you might be confusing absolute or rooted FQDNs (Fully Qualified Domain Name) and relative FQDNs (in the context of DNS) with valid or invalid email addresses.

An absolute FQDN ends with a period:

    apple.com.

A relative FQDN does not:

    apple.com

When a DNS resolver sees an absolute FQDN it has a pretty direct path to getting the corresponding IP address.

If, instead, the resolver is looking at a relative FQDN it does not, and it tries to fix it, and it could go around in circles for a while depending on where the resolver is running. What the resolver does is add different forms of DNS suffixes it has available until it either figures it out, fails or times out. For example, if the resolver is running on www.example.com, it might try the following:

    apple.com.www.example.com.
    apple.com.example.com.
    apple.com.com.

If it finds any CNAME records it will follow them and go around in circles some more. I ran across this recently while looking at the whole issue of email validation through DNS. So, this is fresh pain you are hearing about!

Here's my post on SO:

http://stackoverflow.com/questions/14065946/what-would-cause...

I also have a DNS function test page on one of my sites here:

http://www.tommyteaches.com/test/test2.php

If you try a domain like "notarealdomain1.com" you'll see that it takes a while (20 to 30 seconds per test) for the DNS functions to resolve it. If, instead, you enter "notarealdomain2.com." (period at the end) the DNS resolver will come back almost immediately. This is due to the DNS suffix append mechanism described above.

Note that if you enter the same name twice, the second test will return faster due to caching.

Here's an interesting read:

http://tools.ietf.org/html/rfc1535