We have seen that the three main TCP/IP protocols for email delivery are SMTP, POP3, and IMAP. While SMTP (Simple Mail Transfer Protocol) is for sending emails, IMAP (Internet Message Accessing Protocol) and POP3(Post Office Protocol) are for receiving them. Basically, you can choose between IMAP and POP3 based on your needs. Both have their advantages; however, usually, IMAP suits most people.
The main difference between IMAP and POP3 is that IMAP always syncs with mail server so that any changes you make in your mail client (Microsoft Outlook, Thunderbird) will instantly appear on your webmail inbox.
On the other hand, in POP3, your mail client account and mail server are not synced. It means whatever changes you make to your email account in the mail client will not be transferred to the webmail inbox. POP3 downloads the email from a server to a single computer, then deletes the email from the server.
So, in simple terms, if you are using IMAP and mark a mail as read, it gets marked as read in your web based inbox too (because the changes are happening on the server). However, this won’t be the case if you are using POP3, because the mails are downloaded to your PC and the changes won’t reflect on the server.