When comparing Simon's Mail vs Postbox, the Slant community recommends Postbox for most people. In the question“What are the best cross-platform email clients?” Postbox is ranked 8th while Simon's Mail is ranked 11th.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Supports viewing many attachment types
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.
Cons
Con Web-application
Even when installing local bundles, you don't know where the emails and settings, especially passwords, are stored.
Con Some message types can't be read
Con No support forum
Support for paid product has no online forums.