When comparing Postbox vs Sylpheed, the Slant community recommends Postbox for most people. In the question“What are the best e-mail clients for Windows?” Postbox is ranked 10th while Sylpheed is ranked 12th.
Ranked in these QuestionsQuestion Ranking
Pro Clear interface
Pro Forgotten attachment reminders by default
Reminding the user that they forgot to attach the file if they mentioned one in the body of the email is one of the most useful features of any email client and so many don't include this. Postbox is pre-configured to look for certain keywords (attached, CV, spreadsheet, pdf etc) and you can specify your own keywords in addition.
Pro Blocks tracking gifs
Has the ability to block tracking gifs commonly used for read receipts.
Pro One Inbox for all accounts
All messages in one place. But you can select to show only messages from each account either.
Pro Quick Bar for macros
Quick bar allows you to add a variety of content to your message. It will let you reuse URLS in your Sent folder, and autocompletes against canned responses, signatures, and topics.
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 support forum
Support for paid product has no online forums.
Con No encrypted mailbox
Likely hasn't been done by anyone since Calypso/Courier back in the 00's.