Home Page

VPN & Cryptography


Email & Spam

Security Topics


Email Spam

Zero Day Window


Bayesian Algorithm

Content and Connection control

Directory Harvesting Attacks

Email Encryption

Email Archiving

File attachments

Image scanning

Port forwarding and MX records

Reputation filters

Encrypted attachments

Grey Listing

Email Monitoring

Internal Email Security

Open Relay

Per user quarantine area

Reverse DNS lookup & SPF

RFC Compliant emails


Email Throttling

What is Spam

Whitelists and Blacklists





Simple Mail Transfer Protocol (SMTP)

Simple Mail Transfer Protocol (SMTP) is mainly a server-to-server protocol and is used to send email messages between e-mail servers. The SMTP protocol is a TCP based connection running over port 25.

Email servers such as MS Exchange and email security appliances use SMTP to send and receive email. Clients however, only use SMTP for sending email to an email server. Clients do not receive emails via SMTP, clients typically use POP3 or IMAP to pull down and retrieve their emails.


Post Office Protocol Version 3 (POP3)

POP3 is an application layer protocol and used by email clients to retrieve email from an email server. Most web based email systems, such as Yahoo and MSN, provide POP3 as an additional service to deliver email to end user email device. You can configure your email client application software, such as Outlook, Mozilla, Thunderbird, Eudora, to download your emails on your local endpoint device using your Yahoo or MSN account.

With the use of POP3, the end user’s client software downloads the email message from the email server and stores the email on the local client computer. With POP3 there is no option to leave a copy of the email on the server once its been downloaded. Using POP3, emails are held on a remote server until a client retrieves the email, which is then stored locally on the user's hard drive.

Advantages using POP3

Advantages of using POP3 are as follows:

- Local access to emails means users have the ability to view and edit their emails even if they are offline.

- Storage space is only limited by the end user hard drive

Disadvantages using POP3

Advantages of using POP3 are as follows:

- Email is downloaded locally means you will not have access to those emails on a different computer

- POP3 is slower than IMAP4, since it has to download email messages before you can view them.


IMAP4 (Internet Message Access Protocol)

IMAP is another application layer protocol used for the retrieval of email which has similar attributes to POP3, but has been enhanced in some aspects overcoming some issues with POP3.

With IMAP, the email client is constantly connected to the email server, and does not download or make local copies of the email messages to the local client’s computer. Emails are left on the server, unless deleted by the account holder. IMAP was designed to overcome some of the problems with POP3. Using IMAP enables a user just to download the headings of an email, so they can decide which ones they want to download completely. A synchronisation occurs between the client and server and only displays message header information for emails that are on the server for that particular user.

Advantages of using IMAP4

Advantages of using IMAP4 are as follows:

- You can access your emails from any endpoint device, since all email messages are stored on the server.

- IMAP is generally faster than POP3 as you’re viewing emails on a server

- With IMAP, multiple users can connect to a single mailbox, which is something you cannot do with POP3. IMAPS provides better search capabilities when looking for emails.

- Because only the headings are downloaded initially, users can quickly select which emails they want to download in full, and delete the unwanted emails. This provides ease and faster access, especially for mobile users with limited bandwidth.

- Another advantage of IMAP is an email does not need to be deleted multiple times, which can be a problem in POP3. In POP3, if an email message has been downloaded and you have left a copy on the server, multiple copies of the same email exist, which makes housekeeping a more difficult task.


IMAP4 disadvantages

Disadvantages of using IMAP4 are as follows:

- IMAP is not widely supported, and is better for large business networks.

- IMAP is also a complicated protocol, and needs to be setup carefully.



Depending on how users access their emails, will determine which the better protocol to use is. Users who work from a single endpoint machine and use web based email to access their emails, would be more suited to use POP3. Users who share mailboxes, or access their emails from different machines, would be better suited to use IMAP.


Ports used for each protocol

POP3 uses port 110 and 995 for a secure connection.

IMAP uses port 143 and 993 for a secure connection.

SMTP uses port 25 and 465 for s secure connection.


Further Reading

Wikipedia's guide to IMAP4 SMTP POP3