That's not a standard term. Better ask the one who uses it.
In general, anyone who offers a mail server to a company with the company's domain name and different users in that domain, like mary(at)mycompany.com and john(at)mycompany.com, can use that term.
Common examples are a lot of companies offering 'hosted exchange' or google's 'gmail for work'.
The advantage is that the company doesn't have to run its own e-mail server.
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