Hard bounces and soft bounces
SMTP2GO categorizes all bounces into hard bounces and soft bounces.
We do this categorization by analyzing the error message that is sent to us by the recipient's mail server.
If the message indicates that there is no chance that the email address will work in the future, it is classified as a hard bounce.
Examples of hard bounce error messages, which usually begin with a 5xx code:
The email account that you tried to reach does not exist.
This user doesn't have a yahoo.com account
Examples of soft bounce error messages, which usually begin with a 4xx code:
Recipient address rejected: Mailbox full
The email account that you tried to reach is over quota.
This message was blocked because its content presents a potential security issue.
Our system automatically blocks any attempt to send to an email address that has hard-bounced for a period of 7 days. This is an important policy: if we continually tried to send emails to hard bouncing email addresses, we would very quickly damage our reputation in the email community. After 7 days, the hard-bounced email address will be automatically unblocked and sending can resume.
If there are still issues with the recipient email address, however, it will hard bounce time and time again. You should, therefore, remove hard bouncing email addresses from your local mailing list. We record blocked attempts to send emails and call these 'rejected attempts'. You should aim to have zero rejected attempts.
If you find a hard bounce that you think may work again, you can unblock it from the 'Reports > Bounces' page in your SMTP2GO control panel.
Email addresses that have soft bounced are not blocked by our system, as soft bounces usually indicate a temporary issue that will self-resolve. Our servers will continue to try to send a soft bounced message until either the email goes through or a 'connection timed out' message is generated.