If Mutt is not a traditional mail client, then what is? heirloom-mailx?

I read my mail with less. That's a nontraditional mail client.

Yes.

When I first started in the industry my colleagues told me that I should be using the read-mail program. It has a mode that allows you to use regexps to read your mail really fast. I was told that the normal way to run read-mail with the "really fast" mode was:

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.

Do NOT suggest people try a command which will recursively remove all files ignoring file permissions. Most people will be aware of the hazard. Those who aren't will suffer irreparable harm.

Be careful with which Exchange servers you try offlineimap with. There was a severely bad interaction in Exchange with how offlineimap operates. Exchange used a global database with a small limit (I want to say 2^16) for unique message headers. Offlineimap injected unique headers for synchronization purposes. This quickly led to resource exhaustion and essentially blocked the server.

each email is supposed to have a globally unique Message-Id header, offlineimap or not.

No, this was a unique header name, X-offlineimap-????; not a value. My understanding is the DB table was a list of unique header names.