The Bounce Categories
We can categorize bounce emails in two ways: synchronous or asynchronous. Synchronous bounces are instantaneous as the failed delivery attempt is returned back immediately. An asynchronous bounce is a message that, after a period of time, is returned after the message was sent out.
The way the message is intercepted will determine the speed at which it is returned in a bounce state.
- An email sent to an invalid address, for example, will be terminated. When the receiving server identifies the address as invalid, the connection is cut, and the email is returned: a synchronous bounce.
- An asynchronous email bounce occurs when the receiving server attempts to process the invalid email. The server acknowledges the message and continues to attempt to deliver it until the delivery fails. For example, if a mail server accepts a message and later determines the user does not exist that recorded bounce is asynchronous.
Once an email falls into one of these two categories: synchronous or asynchronous, it is further assigned a type. This is identified in the return email header.
The Soft Bounce
A soft bounce (transient failure) is an email that has been returned back to the sender. The mail is undelivered after it has been accepted by the recipient’s mail server. This is usually a temporary condition with an expectation of clearing up in the future. However, it is good practice to monitor these and remove them from your list. Advanced email tools automatically handle this process.
For example, our software by default will try to send a message three times – the initial send, and two retries. If a mail server reports that an address is permanently non-deliverable, ListManager will only attempt to deliver it once. This is known as a permanent failure or hard bounce. Messages that are undelivered for other reasons, like a full mailbox, are known as transient failure or soft bounce. In that instance, if an address is undeliverable initially, including all the retries it’s considered to have only bounced once.
Soft Bounces may occur when
- An Email is returned undelivered because the receiver’s mailbox is full.
- An Email Message Size is too large to be delivered.
- Autoresponders, such as a vacation or out of the office message may be incorrectly reported as a bounce.
- The connection to the sending or receiving server timed out.
- The connection was refused.
- Network issues.
The Hard Bounce
All things not being equal a soft bounce on one server may be interpreted as a hard bounce on another server. A hard bounce is normally perceived as a long-term or permanent condition. It is generally not expected to clear up any time soon. It is good practice to remove hard bounces when they occur. However, you might want to develop an internal policy to remove the address after a few consecutive returns. Hard bounces may clear up: i.e. temporary system fault or a blacklisted domain.
Hard Bounces may occur when
- The recipient's address is misspelled.
- The user doesn’t exist
- Your domain is blacklisted
Some ESPs such as Dundee Internet offers a more granular bounce grouping. Rather than just a “Hard and Soft Bounce” report, accentuated with a uniquely colorful graph for easy evaluation.
