Thank you for your answer, but it would be easier to me to understand it with a sample.
Currently this is what I have:
the droplet name is something.com (not domain1.com or domain2.com)
it host 2 domains:
domain1.com
with MX: domain1.com.
domain2.com
with MX: domain2.com.

Do you suggest to change domain2 MX to domain1.com.?
In this case emails from user@domain2.com will not be treated as spam?

Your droplet has a dedicated IP. For people to remember this droplet’s address easier, you are asked to enter a ‘hostname’.

What is hostname? This is the part where most people get confused. They assume by hosting a website 'abc.com’ in this droplet, you should enter 'abc.com’ as hostname. Well, there is nothing wrong about it, but it become an issue when you are hosting more than one website in single droplet (who don’t?)

By practise:

Droplet #1: fire.serverdroplet.com
Hosting website 'abc.com’ and 'xyz.com’

Droplet #2: water.serverdroplet.com
Hosting website 'def.com’ and 'mno.com’

Your PTR are assigned to both fire.serverdroplet.com and water.serverdroplet.com

the RFC doesn’t specify that there can only be one PTR per IP; as some one else who would like this control, i’m responding. it’d both save me addressing and time; presently this is a configuration i employ frequently for some things which are mostly irrelevant. what is relevant is it’d make things more useful, as for standards there is nothing in violation of the standard to setting more than one. with that in mind my question isn’t whether it’s in tune with the RFC but whether it’s a configuration DO is willing to permit, that, or classless reverse delegation would be nice. although classless reverse delegation doesn’t really elate me, i’d prefer the ability to directly manipulate the record on the IP for several reasons, not to mention, I frequently am in the habit of assigning an A record in addition to the PTR…that is not usual, but also, should any doubt that is acceptable, do try and start named without having any reverse delegations possessing an A record; bind 9 won’t even load the zone.

This isn’t how you do it, for one a ptr can only be a single domain
Secondly, you should point your second domains mx record to the same A record as domain1’s mx. That way they will both work with the one ptr.