Mail::Spool is a "pure perl" implementation of mail spooling, unspooling
and sending. It is intended to be used with daemons such as
Net::Server::SMTP (to be released soon), but it also contains its own
daemon (based off of Net::Server::Fork) that can be used if necessary.

It is also intended to be used as a quick spooling mechanism for perl
scripts. As it can write straight to the queue without opening another
process.

WWW: https://metacpan.org/release/Mail-Spool