We are hosting some mail accounts/websites with 123-reg and because their TTL is so poor we wanted to utilise dyndns.org to ensure we can quickly change the IP addresses should the worse occur.
This is the domain and associated records,
www CNAME webserver.dyndns.org
10 MX mailserver.dyndns.org
and then at dyndns.org I have the appropriate A records, we want all our domains are set up this way, then should one fail I can just change one record at dyndns.org and it will change quickly as it's TTL is just 60 seconds.
If I email from my hotmail account all works fine but if I email from my yahoo account then I get error message returned. On looking at the debug monitor I see the SMTP session connecting and the line
"RCPT TO: email@example.com"
which obviously does not exist.
Hoever a Hotmail trace correctly shows the line as
"RCPT TO: firstname.lastname@example.org"
Questions really are
1) Can I configure my DNS in this way?
2) How come different ISPs are solving the recipients email address in different ways? Yahoo don't seem to be realising the MX record is a CNAME which needs resolving also.