As stated in the Mobility Devices section:
Sounds like your email server is not being told by your email client to delete messages from the server when downloaded.
If you provide more details, the Neighbourhood folk can better help you.
Please post further details of your inquiry in one forum, and close the other thread.
Each email client that accesses your POP email account remembers the sequence number of the last email message it received, and it requests all messages since that one from the POP server. If it has been a while since you last used a particular email client, it may go back a long ways fetching messages that you've already read in your other email clients.
I addition your email client can tell the POP server to delete the messages after it retrieves them. If it does that, then those messages are gone from the POP server and can't be retrieved by other email clients. So you have to decide for each of your email clients whether you want to delete the messages from the POP server or leave them. For example you might want to leave them if you are reading email on your mobile device so that you can retrieve the same messages later on your home PC and store them there in your email archive. Your POP server has a limited amount of storage allocated for your email, so if you get a lot of email and you never delete your messages, it will eventually fill up. In that case it would usually return a "mailbox full" message to senders.