I started using a webhost that uses cPanel but used an external DNS host rather than contacting my host every time I wanted to make a change to the DNS zone for my domain.
Once I changed my settings with my DNS host for my MX (mail) Records to use that of my webhost, I started getting SMTP error 451 (TEMP_FAILURE: SMTP Error (state 13): 451 Temporary local problem - please try later) whenever I tried sending test e-mails to myself.
I wasn't able to receive e-mail for about a week because of this problem, and my alternative e-mail address was getting messages that e-mails to my primary address were being bounced as undeliverable.
My webhost was not sure how to solve the problem. At first it was thought it was because SpamAssassin/spamd had recently been disabled but when they re-enabled it mail still wasn't getting through.
When I used the "Trace an E-mail Address" option in cPanel it gave the result of "lookuphost via lookuphost" which I thought looked wrong.
So, after 6 days of thinking the problem was because of my DNS MX records with my DNS host, or a difference between those and the MX records with my webhost, I discovered the cause of my problem
For some reason, something got corrupt somewhere so I used the cPanel option "Modify Mail Exchanger (MX Entry)" to change the MX record to thejc.me.uk. and run a trace again.
It gave the result of "virtual_user to local delivery" which is what I was hoping for since that was the same result I got from my sub-domain (for which mail was still working).
When I changed it back to mail.thejc.me.uk. I ran a trace again and was still getting the response "virtual_user to local delivery".
After sending a test e-mail to myself, my e-mails were once again working.
I've posted this as I had to find the soloution myself, since Google didn't have any hits for "lookuphost via lookuphost" which I suspected was the problem, and my webhost couldn't pin down where the problem lied.
Thanks for reading,
John.