There is an ideal number of devices using the same mailbox, which should not be exceeded if POP2 is involved - ONE !!!!!!!
POP3 causes utter confusion and I can never understand why people use it, especially if they are not experts in configuring email.
POP3 was designed as a single email client, long before the concept of a computer in your back pocket was even a wet dream. It works fine if there is a single device (usually nailed to a desk) to where all mail will be received and sent from.
POP3 does not store sent items on the server, they are only on the local device, thus using several devices with POP3 and sending email from each will cause utter confusion as to where that email you sent the other day is to be found.
POP3 only looks at the inbox on the server, it is designed to DOWNLOAD and then remove mail from the server ... which explains why you do not see the same emails on all devices.
Downloaded mail and any placed into LCOAL folders needs to be backed up locally - YOU are solely responsible for the security of your email history.
IMAP is designed to SYNCHRONISE the content of an off-line client with the contents of the server. Email therefore remains on the server until deleted expressly by the user - that is YOU the user remains in full control of what happens on the server. Therefore multiple IMAP clients will see exactly the same as each other.
IMAP clients place sent items into the server's sent items folder, therefore multiple IMAP clients will (generally***) see the same sent emails.
If you please emails into sub-solders, again all clients see the same.
Some people believe that setting up a client as POP3 is the answer for "I do not need to consider" house keeping of their email - just let a POP3 client in the mix harvest emails off the server. As you report, that just causes utter confusion ... people do need to take control of their housekeeping activity. Something we are all rather bad at is failing to cull junk email!
You should consider setting up all email client to IMAP - note that you cannot change a client to IMAP from POP3 and on come clients, you cannot have two email accounts referencing the same email address.
You will have to consider carefully how you save the emails already downloaded on each of the POP3 clients and potentially how you get them back onto to the server. Reflecting on the old Irish advice "If that is where you want to get to (three devices using IMAP having identical views of your mailbox) I would not think about staring from here...".
***Some (rather poor) IMAP clients do make up their own mind which folder on the server to use for sent items (chose from "sent", "sent items" or "sent mail"). Some will allow the user to specify the folder (thereby all clients can be made to use the same one) others will not (for example WIn10 mail) and thus the flexible clients will need to bend to the limitations of the inflexible ones. Remember that Webmail is also an IMAP client which will be impacted by the presence of POP3 clients in the mix.
Superusers are not staff, but they do have a direct line of communication into the business in order to raise issues, concerns and feedback from the community.
