PardsFan wrote:
I don’t know what POP or multiple mail clients are - beyond my level of understanding.
Do you have more than one iPhone or iPad or Mac or Windows or other computer accessing this mail?
If so, you have multiple mail clients.
Mail works using several different network protocols, two commonly used are POP and IMAP.
It'd be nice if this was easier to check, but here's one way:
On iPhone or iPad, Settings > Mail > Accounts > tap the problem Mail account > tap Account > tap Advanced > scroll down, and you should see incoming settings for POP or IMAP. Several lines up from the bottom of that display will be a line labeled Server Port.
Usually POP will show as Server Port 110 or more commonly as 995, and IMAP will show as Server Port 143 or more commonly as 993.
If you see 110 or 995, that downloads messages locally on the first mail client that accesses the messages, and the rest can get an error when they try to access the message.
Switching the connection to IMAP avoids that. If you find 110 or 995 there, I can post how to switch.
No add-ons that I’m aware of.
We have not changed any settings on our devices - problem just seemed to come out of nowhere several months ago
"We"? Are you possibly sharing an Apple ID with somebody else? That tends to get tangled.