How to - one email address checked by two users

You want to use IMAP! POP3 is not designed for this scenario!

While you can tell POP3 to keep a copy of your messages on the server, it is also downloading them to your local coputer. Note the word "copy". Using this option with a mailbox shared with one person, you'll wind up with three copies of every messaage: one on the server, and one local copy on each of your systems!

Furthermore, it's impossible to share Sent messages in POP3. All sent mail is local.

IMAP, on the other hand, will do exactly what you need. While some data for each message is downloaded locally, the server keeps the original of all messages. Every message either of you access is the same.

In addition, all sent mail is kept on the server.

IMAP will give you a truly shared mail box.

Use POP, and have both email clients "leave a copy of messages on the server" for some reasonable amount of time for both parties to have checked it. Choose the options you want in your email client account settings.

If you need to use a single mailbox, then configuring the mail clients to leave messages on the server, as suggested by @Josh, is probably best. That will work equally well whether you use POP3 or IMAP4, since the key is to configure the client to leave the message on the server. IMAP4 works just as well as POP3 in an "offline" setting, even though it's designed more as an online access protocol than is POP3.

However, even though it is going slightly outside the scope of the question as asked, unless you have some very specific usage scenario that requires using a single mailbox, I would actually suggest that you create two mailboxes and then set up an alias on the mail server to deliver one address to both mailboxes. Then, what one user does to the e-mail on the server has no consequence for the other user. I don't think there are any full-fledged mail servers that do not support such a concept, whether it's called aliases, distribution lists, forwarding accounts, or something else.

You'll have to train the users to CC or BCC the common address on any replies, and look for replies before replying themselves, but it completely eliminates the risk of one user deleting e-mails for the other.