I have reread your post and am looking at it through fresh eyes. I think that i missed something and gave you incorrect advice. I read your post as I want to have emails such as firstname.lastname@example.org and somehow totally missed that you are ALREADY using the domain abc.com for the web and email … so, my comments were incorrect. To do what I now think you want to do, which is to create a mail server handling ONLY mail.abc.com, you need to take a slightly different path.
I highly recommend that you PM me with your actual domain name and what hostname you wish to use, then I can give you exact directions.
Basically, if I am now understanding correctly, you have need to handle DNS externally from MiaB for the new mail.abc.com email service. To accomplish this, remove the glue records that you created. They will not be necessary. Do the setup of MiaB as normal, making sure that you follow my advice on creating the initial user email account and confirming the hostname is created properly. Then in your external DNS - whatever DNS you are now using for abc.com - you need to add records for MiaB including the A record mail.abc.com pointing to your MiaB IP. Additionally a MX record for the mail subdomain needs to be pointed to mail.abc.com with a priority of 10. You will also need to carefully add other DNS records as shown in the admin area of MiaB under Settings>External DNS. Make sure that the entries are all for mail.abc.com and not entered incorrectly as being for abc.com.
I am really sorry for the confusion … please confirm that I understand correctly now. And PLEASE feel free to PM me for additional assistance with this.