16.7. Internet Message Access Protocol (IMAP)IMAP, like POP, is a protocol used by mail user agents to retrieve mail, for a specific user from a server. IMAP is a more recent protocol providing more flexibility, including support for multiple mailboxes for each user. POP is commonly used to transfer all messages in a single mailbox to the client from the server; IMAP is designed to store messages on the server, allowing them to be copied and manipulated by the client. IMAP is a much more capable protocol than POP and correspondingly is harder to implement securely.
16.7.1. Packet Filtering Characteristics of IMAPIMAP uses straightforward TCP connections to port 143 and is therefore easy to allow through packet filters. IMAP over SSL currently uses port 993, but an earlier convention uses port 585. Several variants of IMAP are in use (you may see variants described as "v2" or "rev4", for instance), but all IMAP versions in wide distribution use the same port.
ACK is not set on the first packet of this type (establishing connection) but will be set on the rest.
16.7.2. Proxying Characteristics of IMAPIMAP is a straightforward protocol to proxy, since it uses a single TCP connection. There do not appear to be any IMAP-specific proxies available at this time, but generic proxies will work with IMAP (without providing any strong security guarantees).
16.7.3. Network Address Translation Characteristics of IMAPIMAP does not use embedded IP addresses and will work with network address translation without problems.
16.7.4. Summary of Recommendations for IMAP
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