I’m supporting an organisation that has some old Macs which can’t be upgraded beyond MacOS High Sierra. They love the large screens on these iMacs so don’t want to get rid of them. But due to changes in their email hosting setup, they need an email client that can authenticate with OAuth2 instead of standard username/password. Mail on High Sierra can’t do this. Does anyone have recommendations of a good email client that can and will run on such an old version of MacOS?
For the moment, Thunderbird still supports High Sierra and is capable of handling OAuth 2.
Another alternative might be webmail using a browser that still supports High Sierra, like Firefox or Brave.
In any case, the organization should be thinking hard about its migration strategy. The clock is ticking on these systems. It seems pretty safe that Firefox ESR (Extended Support Release) and Thunderbird ESR will keep support for High Sierra through April 2024, but I wouldn’t expect much more than that.
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Thanks, I’ll look into that. They do already have browser webmail access but want a proper client.
As far as migration, I agree! There are only 2–3 of these High Sierra machines left. They would have got rid of them by now, but they’re large screen iMacs and they love the screens. This is the problem with integrated iMacs – the display outlasts the computer. 
But they are going to have to replace them soon one way or another, this is just the latest in a line of incompatibility issues when running such an old OS.
I’d also suggest Thunderbird, but Postbox also supports High Sierra, at least right now, if you want to look at another app. It’s not free, but there is a 30 day trial.
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