Emails will be blacklisted when a contact opts out of your account.

An email address will also be blacklisted when it is deemed to be undeliverable because messages sent to the address consistently bounce.

There are three basic types of email bounces:

1. Mailbox full
2. Auto-reply
3. Undeliverable 

When the recipient's mail quota has been exceeded on the destination mail server, a reply will be sent advising that this message cannot be delivered now but may be delivered later. This type of bounce does not cause an address to be blacklisted. 

An auto-reply is usually configured by the intended recipient and indicates that the message has been received but will not be read immediately. This will not cause an address to be blacklisted.

An undeliverable bounce message is an advisory message from a mail server that a message cannot be delivered to the intended address. This may mean that the email domain (the part after the '@' symbol) does not exist or that there is no recipient (the part before the '@' symbol) at that domain. Often this is because the address is incorrectly entered into our system.

Only undeliverable bounces trigger the automatic blacklisting process. When we receive 3 undeliverable bounces within a 3 month period for any address, it will be blacklisted.