November 2006


POP (Post Office Protocol) is the most widely used and available Internet Standard protocol for email retrieval. This paper looks at email support for mobile devices , shows the benefits of using IMAP (Internet Message Access Protocol) over POP and then considers choices for service providers. Use of a POP/IMAP Gateway is considered in detail.

Use of a POP/IMAP gateway will be particularly useful for mobile operators and mailbox providers, and follows the model of the Open Mobile Alliance (OMA) Mobile Email Enabler described in this whitepaper.

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Why POP Servers are Important

There are two Internet Standard protocols used by email clients to access messages:

  • POP (Post Office Protocol) offers delivery and basic access services.
  • IMAP (Internet Message Access Protocol) offers delivery, online storage, advanced access and synchronization capabilities.

Although IMAP is reasonably widely available, the vast majority of email accounts offer only POP access. As the dominant supported access mechanism, POP mailboxes must be considered in the context of any broadly available mobile email service.

Web email services, such as Hotmail, Yahoo and Gmail are good and popular choices for many users looking to access email from the desktop, or often from many desktop locations such as work, home, internet café, etc. Web access to email from a mobile device is not practical, as the interfaces are not designed for small device screens, and use of (often expensive and slow) bandwidth is poor. These services all offer POP access either as part of their standard, free, package or as part of a paid-for service upgrade but none of them offer IMAP access at any account level. Support of these services increases the importance of POP support.

Problems with Direct use of POP for Mobile Email

POP can be used for "basic" email access from a mobile device, such as a phone or PDA, and this is supported by most (open standard) mobile email clients. By "basic" email we mean access to new messages, which is the most important service for most mobile email users. Although you can achieve this level of service with POP, it comes at the cost of increased network usage relative to IMAP. This cost is not important for desktop access over fast & cheap Internet access, but can be a big deal when accessing email over slower and/or potentially expensive mobile networks. Slower networks will lead to slower User Interface response and an inferior user experience. The following section

Checking for new messages

The client must poll at intervals, either manually or automatically. Polling frequently wastes network resource. Polling infrequently means a delay in getting new messages. The IMAP IDLE command allows a client to wait and be notified about new messages.

Scanning new messages

A key task when examining email is to look at new messages using a scan listing. This information is then used to decide which messages to read now, and which to either delete or to leave for later. Message scanning is usually preferable to automatic download of all messages, due to junk mail (which can be deleted without download), non-urgent messages, and very large messages that would be prohibitively costly to download.

POP has a mechanism to download the message header and a configurable number of lines of the message. This can be used to generate a scan listing. IMAP has an optimized search, that allows for retrieval of the fields to be displayed.

IMAP scanning is 4-5 times faster for typical messages. For a 56kbit connection and typical messages POP could scan 4-5 messages/sec and IMAP 20-25 messages/sec. IMAP will provide a significantly better user experience for this critical function.

Downloading messages

Downloading all or part(s) of the message to the local device for reading and in order to take further action related to that message (forward, reply,etc.)

POP can download messages. IMAP can download messages in chunks, so that a message can be downloaded incrementally as the user reads it. IMAP can read message structure and then selectively access body parts. This can be useful to selectively access parts of messages.

IMAP gives much more flexibility to access messages, which can be very helpful for larger messages.

Message send and deletion

Both IMAP and POP handle message deletion efficiently and support a range of client options for deleintg local messages, leaving messages on the server after download (POP) and deleting server copies of messages.

SMTP (Simple Mail Transfer Protocol) is used by both POP and IMAP clients for sending messages.

IMAP for Mobile Email

As can be seen in the previous section, IMAP performs significantly better than POP when used for basic email access. In addition IMAP offers a number of capabilities that cannot be achieved with POP:

  • Access to old email. Desktop POP clients download and delete mail on the server, so it is not generally possible for a mobile POP client to read messages once they have been handled by a desktop client. IMAP leaves messages on the server, and so this is straightforward. (this isn’t strictly true – the POP client often allows copies of messages to be left on the server but POP accounts don’t usually come with the storage space to make this a realistic long-term proposal)
  • Folders and filing. POP does not provide filing, whereas IMAP enables folders on the server, including shared folders.
  • IMAP and new extensions known as 'Lemonade' provide a number of advanced capabilities and optimizations. These are discussed in our whitepapr on open standards for mobile messaging.

Options for Service Provision

The previous sections lead to two basic conclusions:

  • IMAP (or ideally IMAP + Lemonade) is the best protocal for mobile device access to email.
  • Many email accounts can be accessed by POP but not by IMAP.

We have outlined the various options for email access by mobile devices in the table below, each has its own merits depending on the perspective of the various players.

Solution User View Mailbox Provider View Mobile Operator View
Use a POP mailbox. Mobile email solution is poor. Minimises email storage issues. High levels of traffic may increase revenue.
Use an IMAP mailbox. Could be a good move, but there might be reasons not to change. Could be an opportunity to provide a new customer service and increase satisfaction and/or revenue. Could be an opportunity to provide a mobile-oriented messaging service to customers.
Use Blackberry with a POP backend. Restrictive device choice and usage fees could prove expensive. Generates no revenue for mailbox provider. Could be an opportunity to provide an expensive value added service.
Use an IMAP/POP gateway with a POP backend . Could be a good solution. Would prefer to offer IMAP directly for own clients but might see possible revenue in offering a gateway to other mailbox provider’s customers. Could be an opportunity to provide a value added customer service.

IMAP/POP Gateway and Mobile Email Enabler

M-Box IMAP/POP Gateway

Use of a POP/IMAP gateway is a good approach to providing optimum open standards mobile email access to POP mailboxes. Such a product could operate in two ways:

  • Retrieval mode. Here, the POP/IMAP gateway will download and delete messages from the POP server. This is a simple and efficient approach, which is appropriate when all client access is made through the POP/IMAP gateway.
  • Synchronization mode. Here, the POP/IMAP gateway will access messages on the POP server, but only delete in response to user deletion requests. This more complex approach will be good where access is needed both directly to the POP mailbox and through the POP/IMAP gateway.

Use of a POP/IMAP gateway will be particularly useful for mobile operators and mailbox providers, and follows the model of the Open Mobile Alliance (OMA) Mobile Email Enabler described in this whitepaper. As well as providing the services described here, this architecture has potential additional benefits:

  • Lemonade extensions to IMAP and SMTP to provide advanced services could be introduced by both mobile operator and mailbox provider as a way of differentiating their service offering.
  • It would give mobile operators an opportunity to control the protocol interface between mobile email devices and back-end mailbox services.


IMAP is the best approach for providing open standards mobile email. Use of an IMAP/POP gateway is a good approach to support users with POP mailboxes. Deploying such a product as Isode's POP/IMAP Gateway of this nature could provide advantages for users, mail providers and mobile operators.