>I'm a Thunderbird user, but I don't really think that there's a clear winner in that comparison. Outlook has features that Thunderbird has been missing for over a decade, like reliable send-in-background (and other database operations in background unlike Thunderbird that likes to interrupt you to ask permission) and a far more mature calendar product.

A fair point. As someone who has used both Outlook (professionally) and Thunderbird (personally) for decades (IIRC, the oldest message in my Thunderbird email store is from 1996), calendaring in Thunderbird isn't as robust as in Outlook and some of the other weaknesses you mention are absolutely valid.

However, I'd say that many of the advantages of Outlook that you mention are more related to better integration of Outlook into Exchange back ends than to the Outlook client.

If Exchange had better IMAP support and appropriate plugins for Thunderbird, Thunderbird would be vastly superior to Outlook in most respects.

As it is, Thunderbird is already vastly superior to any web-based MUA[0], including OWA[1].

[0] https://en.wikipedia.org/wiki/Email_client

[1] https://en.wikipedia.org/wiki/Outlook_on_the_web

Edit: Clarified Outlook/Exchange integration vs. Outlook as client.

I tend to really harp on send-in-background because I view it as a core feature of an MUA, but as far as I know it's still hidden behind an about:config flag in Thunderbird because it has an excessive number of known bugs, and it's been this way for many years. Another ongoing pain with Thunderbird is the inability to switch between HTML and plaintext when composing a message. Outlook lets you do this, in Thunderbird you have to copy the message, discard it, start a new one in the right mode and paste.

On the other hand, yes, Outlook Web Access is hilariously bad. It's hard to understand how Microsoft flubbed it so bad considering that their consumer outlook.com has a radically better webmail. Different teams, obviously, but you'd think they would have shared notes.

All in all, it feels a lot to me like Microsoft had a really good MUA in 2003 and hasn't done much with it since, while Mozilla had a not-quite-done-yet MUA in 2003 and hasn't done much with it since. Both feel bolted together out of spare parts but in a way that's subjectively a bit different.

Outlook.com and the current version of OWA in Microsoft 365 are nearly the same. What are you still missing? (If you’re on-prem still, you’re probably running a much older version)

Not to mention replies listed in threads. You have to move sent messages to the inbox to get this to behave correctly in TB.

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