Set up a mail server on your own hardware and hand your clients proper email addresses on their own domain - no separate mail provider to pay for or manage.
Turn on sending and receiving on a server, and mailboxes become something you can offer as part of a plan. Each one gets its own storage limit, so a full inbox on one account never affects another.
Every mailbox comes with a web-based inbox, so a client can check their mail from a browser without setting anything up.
Moving a client from another mail provider doesn't mean starting from zero - existing mail can be imported straight in.
Sending, receiving, and the protocols behind them can each be switched on or off separately - a server doesn't have to run the whole stack if it doesn't need to.
A certificate for the mail server is generated automatically the moment it's built, so connections are encrypted from day one - nothing to request, configure or forget.
Email accounts aren't a separate add-on to negotiate - the number of mailboxes a client gets is simply part of the hosting package they're already on, same as their storage or site count.
Creating a mailbox, changing its password, deactivating one a client no longer needs, or jumping straight into webmail - all of it happens from the same place you manage everything else.