Here are steps for mail migration:

  1. will need to record the IP address of the current mail server along with the following settings:
       * ip address
       * pop3 or imap (imap preferred)
       * whether login is by username or username@realm
  2. You, as the ISP administrator, then need to make sure that all TTL records (www, mail, mx, etc) for the domain are changed to a value of 600, up to one week but no later than 72 hours prior to the mail copy. This is done on the DNS server managing DNS for the domain. If DNS is switched to prior to this, can do this step for you.
  3. Then, on the day of the mail transfer, change DNS to IP addresses and MX records defined by (see the list here). The IP address will vary depending on the server you are being installed on, while the MX records are always as follows:
       * 10,
       * 20,
       * 30,