If you still don’t know it, Gmail is now offering free IMAP on every email account! 
Two easy steps:
- Enable it on your Gmail settings:
Click on Settings (at the top of any Gmail page), then click Forwarding and POP/IMAP, then select Enable IMAP and Save Changes.
- Configure your mail client (PDA, Outlook, Thunderbird…):
Basically, the settings are:
Username: your_username@gmail.com
Incoming server: imap.gmail.com Port: 993 Security: SSL
Outgoing server: smtp.gmail.com Port: 587 Security: TLS
If you don’t know what’s the difference between POP and IMAP, have a look here.
Briefly, unlike POP, IMAP offers two-way communication between your web Gmail and your email client(s). This means when you log in to Gmail using a web browser, actions you perform on email clients and mobile devices (ex: putting mail in a ‘work’ folder) will instantly and automatically appear in Gmail (ex: it will already have a ‘work’ label on that email).
Source - 来源:
中文：Gmail IMAP的应用技巧