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If you plan to upgrade to macOS Sonoma later this year but also happen to make use of legacy Mail plug-ins, you're going to be in for a bad time. That's because the update will remove support for those plug-ins for the first time.
Plug-ins can be used for a variety of things, like bulk email management and automation. They can often be used by businesses and power users and now it appears that they're going to have to consider upgrading to something newer or ditching those plugins and their features altogether.
With Apple now putting the macOS Sonoma update through what is sure to be months of beta tests proper to a September or October launch, there is plenty of time for those decisions to be made. One other option is to just not update yet, but that's less than ideal and no use at all if there are new macOS features that you could make use of.
Legacy Mail
As MacRumors points out, Apple confirmed that it would kill off legacy Mail plug-ins previously, but didn't say when that would happen. The warning was given when macOS Monterey was released in 2021, and there was even a replacement for legacy plug-ins announced at the same time.
That replacement is the MailKit framework and developers have been tying into that ever since. However, there is still a good chance that some legacy plug-ins haven't been updated, leaving their users in a difficult position.
Some developers are already aware that Apple is deprecating legacy Mail plug-ins and are working to get their projects ready for the big day.
PSA: Confirmed in WWDC labs that legacy Mail plug-ins are NOT supported on macOS Sonoma. MailKit-based extensions are the only supported method going forward.AltServer’s Mail plug-in will no longer work once you update, but we’re working on a new method we hope to release soon.June 14, 2023
However, while MailKit extensions can be more secure than their legacy counterparts, they don't have the same level of access which could impact the features they offer. As a result, it's also possible that some legacy plug-ins simply won't make the move over to MailKit at all.
Apple announced the macOS Sonoma update during the WWDC event on June 5, but it was far from the only update previewed. The new iOS 17, iPadOS 17, tvOS 17, and watchOS 10 updates were also shown off for the first time — they're all now available in beta and should be ready for the public this September.
That was just the software, too. Apple announced the first non-Intel Mac Pro, an updated Mac Studio, and the first 15-inch MacBook Air during the same event. The biggest announcement was undoubtedly the arrival of the Vision Pro AR/VR headset, however.