Quarantine is a process that temporarily prevents messages to be sent to a contact.
There are 2 types of quarantine:
Standard quarantine due invalid email addresses
Quarantine resulting from an invalid email address is automatic and always active in the application.
This process temporarily prevents messages to be sent to contacts whenever the destination server detects invalid address. The quarantine process gives the contact a status that prevents messages to be sent without deactivating the contact permanently. Therefore, as long as the contact is in quarantine, no messages can be sent, but when the quarantine is lifted, the contact's initial status is restored and messages can be sent again.
There are some rare cases where the email address is placed into quarantine even when the address is valid - for example when the destination server has temporarily deactivated an account or was experiencing difficulties at the time of mailing. If you are aware that a message can be delivered once the remote server services have been restored, you can manually update the contact's status.Never lift the quarantine on an invalid address, as this could have a negative impact on your reputation as a message sender, which is used by destination servers and anti spam services to determine if you are reliable.
See Lift the quarantine.
Quarantine due to too many repetitive delivery errors
Although the standard quarantine allows to clean up your contact list by temporarily deactivating contacts with an invalid address, you can further clean it up by also quarantining contacts who register too many delivery errors over a given period. This additional quarantine option is optional, thus strongly recommended as it will help reduce your delivery error rate and improve the delivery of future messages to be sent.
When enabling this option, you will have the possiblity to specify how many delivery errors are needed to cause contacts to be quarantined, and the period over which these errors must occur.