You keep plaintext archives and drafts of your messages, because that's a fundamental feature of email clients going back 3 decades.
To my knowledge, mutt doesn't store decrypted archives. Drafts are stored in /tmp which can be a filesystem stored in RAM. I think using Mutt also takes care of your second point.
Your third point is the biggest problem with any system where security is bolted on (e.g. SMTP, POTS, etc.) - your end may be secure but your interlocutor is liable to compromise you one way or another. Though, as you say, this isn't a fundamental property of all encrypted messaging systems.
To my knowledge, mutt doesn't store decrypted archives. Drafts are stored in /tmp which can be a filesystem stored in RAM. I think using Mutt also takes care of your second point.
Your third point is the biggest problem with any system where security is bolted on (e.g. SMTP, POTS, etc.) - your end may be secure but your interlocutor is liable to compromise you one way or another. Though, as you say, this isn't a fundamental property of all encrypted messaging systems.