mx records?

[David]

We are testing a new mail server.  Currently, our new mail server is our
primary MX record..which gets email, then forwards it to home.domain.com, to
create all our users (so we don't need to do it by hand).  When we switch
the mail CNAME, the new mail server software runs fine for a few minutes,
then freezes..

I tried telnetting to the newmail server (port 110) and doing
USER username
PASS password
QUIT
those work quickly

If i were to do a RETR or LIST command, the mail server software would
freeze.. and there would be no way to disconnect... when you try logging in
again..it says mailbox in use.  Is there a loop going on? or is it software?
the CPU and RAM usage are normal... pinging the server works.

newmail.domain.com: NT
domain.com.: Worldgroup

BEFORE SWITCH (everything works)
----------------------------------------
domain.com.    IN    MX    10 newmail.domain.com.
domain.com.    IN    MX    20 domain.com.

home.domain.com.    IN    A    1.1.1.1
domain.com. IN A 1.1.1.1
newmail.domain.com.    IN    A    1.1.1.2

mail.domain.com. IN CNAME domain.com.
-------------------------------------------

after the switch (mailserver freeze)
----------------------------------
domain.com.    IN    MX    10 newmail.domain.com.
domain.com.    IN    MX    20 domain.com.

home.domain.com.    IN    A    1.1.1.1
domain.com. IN A 1.1.1.1
newmail.domain.com.    IN    A    1.1.1.2

mail.domain.com. IN CNAME newmail.domain.com.
--------------------------------

just the CNAME change... and it stops working correctly.
what you think? DNS or Software?

any help is appreciated
  - David