Ronny - with all due respect, I don't think you understand how DNS is involved in the delivery of E-Mail.
MX records and A records are nothing to do with each other, and you can easily have A records for www.somedomain.com and somedomain.com pointing to the same webserver (configured with name-based virtual hosts and therefore hosting hundreds of domains), while separately having the MX record for somedomain.com pointing to a mail server (that could again handle mail traffic for multiple hosts), allowing the mail to be delivered regardless of what the A record(s) point to.
That said, if the domain doesn't have an MX record, then most mailers will attempt to send to the server pointed to by the A record (I think). But otherwise it doesn't matter.
As you say, of course, there are plenty of other services that are hosted on the internet, and indeed we might have "ftp.somedomain.com" and various others. But as most people (certainly in the general public) want the website, it makes sense to have the truncated "somedomain.com" be assumed to be "www.somedomain.com".