It's a funny trick of the english language that when we say someone uses an email client, it comes across as an exclusive use. We need an explicit patch.
My presumption, based on your defensiveness/evasiveness, is that the answer is "yes."
you can send email using external programs (local smtp server and openxchange for exchange).
Mutt is not a traditional mail client, it is more of a glue between multiple programs so you can have one interface for all of your emails need.
I read my mail with less. That's a nontraditional mail client.
How? Sounds like you just grep your way through the mailbox?
rm -rf *
Disclaimer: For the Unix impaired, this is an "expert only" utility. I am not responsible for the loss of all the files in your home directory should you attempt to use it.
In large implementations (such as Exchange Online in Office365) they may not support some of those services. IMAP can be very resource intensive and bog down a CAS or storage server.