If Thunderbird gets rewritten I can imagine at least for some time most addons will not work anymore. I am sure a mailprogram doesnt need to be an entire browser, but changing its foundation, while Addons are already pretty few, could strip many of its needed features DKIM verifier: this is essential and should always be integrated. It works on nearly all mailservers. No reason not to integrate it by default Some carddav support for syncing contacts with Android. K9mail works perfectly like that, Thunderbird should allow syncing from a local .vcf file too Reply with Subadress: essential when using aliases and not wanting to accidentally use the wrong alias on a mail Thunderbird conversations: This will hopefully be completely integrated, not only showing received but also sent messages. Mails are used like Chat very often, and not having threads is horrible. Simple Template, mail templates. Mail programs like outlook for sure support this. Mails always look the same, especially if you are writing from a business with complex templates. Having this in TB is essential Send Later: This addon is very broken but solves an important feature currently standing out from Mailspring. Check if recipient viewed mail content: I dont know if this is possible without servers and loaded images, but any form to integrate this would be really helpful. Also a feature of mailspring. Thunderbird is the best Mail client on the Desktop! But it needs to tidy up, fix its horrible UI, get modern and integrate essential features.
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