I am working part time for a small company, they have about 40 employees that use the email everyday for work and recently they have acquired a MS account for 10 employees that use it mainly for teams with customers but also sharepoint, etc.
To buy an MS account for each of the 40 would be too expensive and necessary because the other 30 only really use email in the day to day work.
Or, you could probably just get some just-Office licenses for the 10, and keep all your email on the secondary server. Trying to relay half your mail sounds like a mess.