postfix-cdb 3.5.6-1ubuntu2 (armhf binary) in ubuntu impish

 Postfix is Wietse Venema's mail transport agent that started life as an
 alternative to the widely-used Sendmail program. Postfix attempts to
 be fast, easy to administer, and secure, while at the same time being
 sendmail compatible enough to not upset existing users. Thus, the outside
 has a sendmail-ish flavor, but the inside is completely different.
 .
 This provides support for CDB (constant database) maps in Postfix. If you
 plan to use CDB maps with Postfix, you need this.