These days email is ubiquitous.
We expect it to be everywhere, work on everything and communicate instantly.
Unfortunately, that's not really how it was meant to be used and the game we are playing today is kind of a cludgey evolutionary catch-up.
But email isn't going away because, well, it IS pretty good really.
Most of us rely heavily on email and when it goes bad we feel quite some pain.
These days many people have one or more email addresses operating across more than one device.
The trick is to set them all up correctly so that they aren't tripping over each other and giving you weird results.
They need to be set up consistently across all devices despite the fact that the devices will probably be running different operating systems, different email app's/programs, talking different protocols and will each have their own "smart" way of configuring emails accounts.
Factor into this that you email provider is quite likely to have changed, or will change, their email server settings on at some stage and you can quickly see this isn't as simple as we all may have hoped.
The simplest way to describe it is for you to imagine two people trying to do the same job at the same time but they speak different languages and each have their own understanding of what the objectives are.
Some things may get done this way. Some things will get undone. Some things will get done twice and some things will trip over each other and not get done at all...and every other possibility you can imagine as well.
If you are finding that emails are showing up one one device but not another, disappearing from a device despite you not having deleted them, showing up multiple times despite the fact you've already dealt with them....and so on, then you should probably contact your IT person.
Ask them to rationalise your email accounts and clients. Get them to make sure they are all playing the same game.
These days synchronisation is the name of the game and you need to understand the ramifications of this.
It is brilliantly convenient and means that you only have to handle email once. But it also means that what you do on one device is automatically reflected on all other devices i.e. if you delete an email on your phone it WILL be deleted everywhere.
Again, ask your IT person to explain if you need more details.
For a long time I've been advocating the storing (read backup) of your important emails outside of your email environment (ref: https://www.ihatemypc.com.au/post/2018/11/14/manage-email-backups-better-drag-and-drop-them-to-folders).
You can probably see why this method of backing up important emails is even more important than before i.e. you need them where they can't be accidentally deleted by a remote device...especially if you share that remote device with someone else.