Bracket emails are removed from the system by default after 1 year. You can view the "Can You Make a Bracket Email Expire Sooner Then One-year" article for more information. You cannot extend the retention period beyond 1 year in the Bracket system. This is due to compliance requirements for most industries being 1 year of email retention.
Archiving Bracket Emails:
You can archive the emails from Bracket if a longer retention period is needed then the default 1 year period. This would require SecureStore or another 3rd party archiving service.
You can add a 3rd party journaling address in the console under the "Encryption" tab under the domain.