Rainmail Server User Guide
Email - POP3 Vs. IMAP

Rainmail Server allows Email clients to work either with POP3 or IMAP protocol. POP3 protocol is resource efficient, but does not have a lot of features. IMAP uses more resources than POP3, but provides more user level features. Unless resource (in terms of RAM, Processing speed) is an issue, it is recommend that your Email is configured using IMAP. 

In this section, a comparison of POP3 versus IMAP protocols is presented. This will help you decide on what protocol to suggest to your users for accessing Email. It is also possible to have one set of users use POP3 and another set use IMAP for accessing emails.

Access of emails from multiple clients (like Mobile devices, Webmail, Outlook, etc.) 
POP3 is not recommended. POP3 is an efficient protocol, but is not specifically designed for multiple client access.
Some of the problems one may encounter are:
  • Duplicate emails (emails downloading more than once)
  • Missing emails (Emails downloading to one client, but missing from another)
  • Corrupted mailbox
IMAP is recommended 
Accessing of emails only from one client and no need to keep a backup on server. 
POP3 is preferred as it takes up less resource 
IMAP will definitely work, but requires more resources 
User needs to synchronize their folders in Email client as well as Server. For example, if user sends an Email from client, it should also appear in Sent Items in the server (in addition to Sent Items in client). 
This is not possible with POP3 
IMAP should be used for this requirement 
There should be a backup of emails in the server. For example if client machine crashes, they should be get their emails from server without any mail loss. 
POP3 cannot be used. POP3 works on a single folder. 
IMAP is recommended for this requirement. IMAP folders can exist both in client as well as Server and if client crashes, can easily be synchronized back from server 
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