[grugq]

To elaborate, the security precautions you take are entirely dependent on the threat you face (or believe you face). If you're Dread Pirate Roberts, creating a detailed diary of you criminal activities is a terrible idea [1]. If you're a 13 yr old girl, creating a detailed day by day diary is probably not a life threatening decision.

You should take the precautions appropriate for your threat model and risk appetite. I suggest deleting correspondence after a set period of time. I know people that do a yearly purge of all their sensitive mail for the previous year in January.

If you are encrypting for the sake of encrypting, as some people do (see: cover traffic), then you can keep archives of your mail because you face no negative outcome if your archive is compromised. There is no downside, so go for it.

[1] https://www.youtube.com/watch?v=pBdGOrcUEg8