Compatibilty between Mojave and Catalina

We know that Catalina is all 64-bit and that Mojave isn’t. We know that there is an incompatibility between Reminders in the two versions. But are there any other incompatibilities? In particular, is Mail compatible across the two versions?

I have a specific interest in Mail. I use iCloud to share Mail. This means that all the mail configuration info, and all my mail messages that I have moved out of IMAP and onto my Mac are also all held in iCloud. This means that I can look at my mail from either my MacBook or my iMac and see the same things. With both those Macs running Mohave that seems to work without any issues.

I need to know what will happen if I update ONE of my Macs to Catalina, but leave the other running Mojave …

I’ve been running High Sierra, Catalina, and iOS 13 and 14 beta without any Mail issues, but I haven’t tested it systematically.

For the past couple of weeks I’ve had one Mac on Mojave and one on Catalina. No discrepancies between the two in Mail (I use FastMail as my IMAP server), Calendar, Messages, iCloud Drive, or Dropbox. No problems at all, really. I intend to move the lagging Mac to Catalina later this week.

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The Mail app on my Mac running Catalina downloads and sends email. It doesn’t sync the read or deleted status on my other devices. Also, when I send email through my account on Outlook 365, the first email usually bounces and requires me to select the Outgoing server.

No idea why this is happening, but neither problem arises if I use the Outlook app.

Because of a change in the way the messages are stored, Houdah Spot cannot search Messages in Mail and Mail Hub does not work.

Houdah Spot does search messages in Outlook, but doesn’t show the Quick view.

Photos in Catalina are incompatible with Photos in Mojave.

what leads you to say Photos isn’t compatible?

A question: had you kept Reminders in the iOS 12/Mojave-compatible format up until now? And if so, did you find that iOS 14 retains the option to refrain from updating them? My Mac is still on Mojave and I don’t plan to install the Big Sur beta - I’m thinking about installing the iOS 14 Public Beta but don’t want to lose reminder syncing between Mac and iOS for the next several months.

I did keep Reminders in the old format. I haven’t opened Reminders on the iPad that’s now running the iOS 14 beta because

  1. It’s not my habit to use Reminders on that device.
  2. I’m afraid to do so.

Because Catalina is working so well on my home iMac, I’ll be upgrading the office iMac to it soon and can then forget about old and new Reminders differences.

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Opening Reminders in iPadOS 14 pb2 gives an upgrade warning then lets you keep using it in the old format, like iOS 13 did.

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Oh, excellent - thanks, good to know.

Hmm. When you do upgrade your other Mac, could you try opening Reminders on iOS 14 - once there’s nothing to lose - to see what happens before updating them on the Mac?