Seamless email transfer?

When I transfer hosting to DH the email account(s) go with it. I need to make this as seamless as possible. When I set up a single user email, I made that person an (FTP) user on the account. Then I was able to choose an existing mail box and avoid the mxxxxxxxx login ID (so the user didn’t have to change anything on their client email program). Now the client keeps everything the way it is, such as and myname is the login id and not mxxxxxxxx.

That worked well. In other words, once DH was hosting the mail the user didn’t have to change anything in his email program on his computer.

If I want to do this for each person who uses email on the account I am transferring from, do I have to go to the trouble of making every one of them an FTP user in order to avoid the mxxxxx login ID? Is there a better way?

Thanks. It does work as long as the whole email address is in the login field. Now my question is, is that standard for other hosts?