Unsync Mail from separate devices

POP protocol Removed the message from the Mail Server.

Thereby two things occur

1 - The Message become Local and on the First Device to pickup the emerge

2 - Any Other device using the same configuration will Never see the message

IMAP will leave the message on the Mail Server

Thereby

1 - Whatever action where done on the First Device that received the message will be reflect to all the Other Devices using the same configuration

Personal Example if I may share using IMAP Protocol

Your reply above, was first seen and marked a Read on my iPhone

I come to my computer and it still appears in the inBox and is marked as Read

If I were to Flag your e-mail with say the Green Flag that would appearing on by iPhone as Flagged Green

What protocol works best in your situation, depends on what the Net Result you what to achieve

Tom from Frankfort wrote:

Up until approx 1 month ago, our family Fastmail email account which is managed through Apple Mail would download to each of our three devices - iMac, Mac Book Air, and iPhone. Whatever actions occurred on one would not occur on the other, being read, deleted, or placed in a file folder - but all would be downloaded from the server to each individual device. For some reason, they are all syncing which has become problematic. They are all IMAP, the software is up to date, and I've deleted and reloaded each of the Fastmail accounts on Mail

From the Bold Text above - the description appears to Not IMAP Protocol but POP Protocol

Reading from the Fastmail site, it appears there can be more than One Admin Account on this Service.

Is it possible another Member has changed from POP to IMAP protocol ?

Tom from Frankfort wrote:

Up until approx 1 month ago, our family Fastmail email account which is managed through Apple Mail would download to each of our three devices - iMac, Mac Book Air, and iPhone. Whatever actions occurred on one would not occur on the other, being read, deleted, or placed in a file folder - but all would be downloaded from the server to each individual device. For some reason, they are all syncing which has become problematic. They are all IMAP, the software is up to date, and I've deleted and reloaded each of the Fastmail accounts on Mail

That is the way IMAP works. All devices logging into the IMAP account synchronize.