Although I agree with Uno's comments, I would suggest that, if you have the expertise to run your own email server, then now is a good time to take control of the DNS yourselves (independent of Company A or Company B)
You should do this first, and then you will always have full control of your domain name. Your company's domain name is a very valuable asset and should not be left in the control of the same company who host the website.
I use an account with Heart Internet to manage all of my domains and I have complete control of all of the zone records, etc. (there are, of course, others who provide this service)
If you are not happy with taking full control of the detailed Zone records, then you can go for the much simpler solution using the same account (with Heart, or elsewhere) and you then change just the name server records to point wherever (and whenever) you want them.
If you are really serious about your business, then you should at least have two Name Servers at different physical locations. If the name servers are all on one server and it fails, you just vanish totally from the Internet. (website, email, everything)
Line One:- Zen - DrayTek Vigor 2600VG
Line Two:- EntaNet (Aquiss) - DrayTek Vigor 2600