How can i manage emails from a different droplet?

HI, I checked the other posts on the website but didnt really find something that could help. I am managing one of my websites from one droplet and i created another droplet to manage the first droplet’s emails. Like lets call the main droplet: droplet A and the mail server is droplet B

so I want all droplet A’s emails handled by droplet B. What I just did is that I added a new domain as mail.mainsite.com and Droplet’s A’s DNS is configured to forward all visits of mainsite.com to its ip address… actually everything is working fine with droplet A. However, droplet B is the issue. i am not sure if the entries are right. I also added an mx entry to droplet A’s dns records that says hostname: mail.mainsite.com and priority 1 I also have virtualmin installed and I dont know if that will make any difference. the hostname of droplet B is mail.mainsite.com.

I really dont know what I am doing so please help. Thanks a lot! :)