It’s notoriously cryptic, undercommented, and heavy on idiosyncracies, particularly the macros.
Nevertheless you can learn a lot from deciphering djb’s source, particularly with regards to defensive programming, so don’t be afraid to dive in. Just recognise it wasn’t necessarily engineered for ease of collaboration. Once you get a feel for the internal structures you may be surprised how clearly you can reason about it.
Perhaps it’s a matter of taste but I always preferred working on Wietse Venema and Timo Sirainen’s code.
Indeed, and Venema's Postfix was a huge win for sysadmins at the time who were sick of sendmail and didn't like the "half required batteries included" approach of qmail, eg. dealing with spotty support for Maildir in other software.
Nevertheless you can learn a lot from deciphering djb’s source, particularly with regards to defensive programming, so don’t be afraid to dive in. Just recognise it wasn’t necessarily engineered for ease of collaboration. Once you get a feel for the internal structures you may be surprised how clearly you can reason about it.
Perhaps it’s a matter of taste but I always preferred working on Wietse Venema and Timo Sirainen’s code.