https://www.fastmail.com/help/technical/imapvspop.html
POP is a very simple protocol that only allows downloading messages from your Inbox to your local computer. Generally, once transferred, the email is on your local computer and removed from FastMail.
IMAP is much more advanced and allows you to see all your folders on FastMail. You can quickly view subjects and message bodies of emails. It can delay downloading larger emails, such as those with attachments, until you want to view them in their entirety. IMAP also allows you to synchronise mail folders between your home machine and FastMail on the web, so that you see the same folders and messages wherever and however you access your email.
Differences at a glance
| IMAP | POP | |
|---|---|---|
| Flexibility | Can view just message headers, and then choose which messages to download. | Have to download all messages at once. |
| Delete or move a message without having to download it. | Must download all messages. | |
| Download only the body of a message. | Must download entire message, including attachments. | |
| Synchronisation | View messages in all folders. | Only view messages from the Inbox. |
| Changes made via our web interface or email software stay in sync everywhere. | Once downloaded, changes can only be made on local email software. | |
| Access messages at home, work, and through the web. | Access messages only from a single device. | |
| Safety | All messages kept with multiple redundant copies on FastMail's servers | Once downloaded, the message only exists on your local computer. If it crashes, the message is lost. |