Email Delivery Issues

Email Delivery Issues Class Notes

Email in the Internet

Email has been around since the beginning of the Internet. As with ordinary postal mail, email is an asynchronous communication medium.

Internet mail system has three major components:

  • user agents (outlook..)
  • mail servers
  • SMTP, Simple Mail Transfer Protocol


SMTP is at the heart of email, is much older than HTTP. It is a legacy technology that possess certain archaic characteristics. e.g., it restricts the body of all mail messages to simple 7-bit ASCII, which was made sense in the early 1980s. It requires binary multimedia data to be encoded to ASCII before being sent over SMTP.

SMTP is used where the sender’s mail server connect to recipient’s mail server. It opens a TCP connection to an SMTP server. After some initial SMTP handshaking, the SMTP client sends message into the TCP connection.

Comparison with HTTP


  1. HTTP is mainly a pull protocol — someone loads information on a Web server and user use HTTP to pull the information from the server. The TCP connection is initiated by the machine that wants to receive the file. SMTP is a push protocol — the sending mail server pushes the file to the receiving mail server. The TCP is initiated by the machine that wants to send the file.
  2. SMTP requires each message to be 7-bit ASCII format.
  3. How a document consisting of text and image is handed. HTTP encapsulates each object in its own HTTP response message. Internet mail places all the message’s objects into one message.

Mail access protocols

POP3 — Post Office Protocol — Version 3

An extremely simple mail access protocol, with limited functionality.

IMAP — Internet Mail Access Protocol

POP3 doesn’t provide any means for a user to create remote folders and assign messages to folders. IMAP is more complex and provides more functionalities.

Email Delivery Issues

The need for this topic is to:

  • Bounces
  • Complaints
  • Suppression list


If your recipient’s receiver fails to deliver your message to the recipient, the receiver bounces the message back to Amazon SES. There’re two types of bounces:

  • Hard bounce: A persistent email delivery failure. e.g., the mailbox doesn’t exist. Amazon SES doesn’t retry hard bounces, with the exception of DNS lookup failures.
  • Soft bounce: A temporary email delivery failure. e.g., the mailbox is full, there are too many connections (also called throttling), or the connection times out. Amazon SES retries soft bounces multiple times.

Bounces can also be synchronous or asynchronous:

  • A synchronous bounce occurs while the email servers of the sender and receiver are actively communicating (hard bounce)
  • An asynchronous bounce occurs when a receiver initially accepts an email message for delivery and then subsequently fails to deliver it to the recipient. (soft bounce)


When user hit “Mark as Spam” button, the email is forwarded to ISP. Additionally, most ISPs maintain an abuse address where user can forward unwanted email messages and request that the ISP take action to prevent them.

In both of these cases, the recipient is making a complaint.

Suppression list

The suppression list is a list of recipient email addresses that have recently caused a hard bounce for any Amazon SES customer. Like any hard bounce, suppression list bounces count towards your sending quota and your bounce rate. If you’re sure that a email address is valid, you can submit a removal request.

Be proactive

What is the CAN-SPAM Act?

Be careful when you send email, make sure it’s formatted correctly, prevent from triggering spam filter and going into Junk.

Verification and Authentication

To maintain trust between ISPs and email service, email service needs to ensure that its sender are who they say they are.

Two methods of authentication:

  • Sender Policy Framework (SPF)
  • DomainKeys Identified Mail (DKIM)
We need to implement both of them, not either of them.

Read more


  • CSYE 6225, Tejas Parikh, Northeastern University
  • Computer Networking A Top-Down Approach (James Kurose & Keith Ross)