IncrediMail is creative, advance, multimedia rich feature email client to share or manage creative mails data while open-source, free, cross-platform, program to manage news, emails, feeds, news-groups, chats, etc. It suits creative users while it is suitable for personal & professional users. It supports Notifiers, animated effects, sounds, emotions, multimedia attachments preview, letters, Ecards, etc. Almost all features are related to compose creative mails while Thunderbird supports mail account set-up wizard, personalized email address, one-click address book, attachment reminder, multiple channel chat, Tabbed emails, web searching option, message archive, activity manager, large files management, Add-ons manager, smart folder, phishing protection, etc.
IncrediMail supports IMM, IML files to save emails mailbox while Thunderbird supports .mbox extension to save or manage message data. After looking at both email client, we can easily say that both are different emails app from each other yet there are thousands of users who are using IncrediMail and also Thunderbird.