Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client.
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No issues with stability or stuff going missing.
Pro Comprehensive settings
Pro Very very lightweight
It is simple and very lightweight where claws mail added a lot of bloat like themes and plugins slypheed keeps it simple.
Con No encrypted mailbox
Likely hasn't been done by anyone since Calypso/Courier back in the 00's.