A simple mail "spool and send" daemon written in Go
go build spoolgore.go
# send every mail spooled to /var/spool/yourapp via example.com smtp service spoolgore -smtpaddr example.com:25 /var/spool/yourapp
# send every mail spooled to /var/spool/yourapp via foobar.it smtp service using plain authentication spoolgore -smtpaddr foobar.it:25 -smtpuser kratos -smtppassword deimos /var/spool/yourapp
# send every mail spooled to /var/spool/yourapp via example.com smtp service, do not try for more than 30 times on smtp error spoolgore -smtpaddr example.com:25 -attempts 30 /var/spool/yourapp
JSON status file
During its lifecycle, Spoolgore constantly updates a json file with its internal status. You can simply view that file to understand what is going on. By default the file is stored as .spoolgore.js in your spool directory, but you can change its path with the -json option.
SIGURG -> suddenly re-scan the queue
SIGHUP -> reload the json status file
SIGTSTP -> block queue scan, useful for manually updating the json status
Sending e-mails from your app via remote smtp can be a huge problem: if your smtp service is slow, your app will be slow, if your service is blocked your app will be blocked. If you need to send a gazillion email in one round your app could be blocked for ages.
"spool and send" daemons allow you to store the email as a simple file on a directory (the 'spool' directory), while a background daemon will send it as soon as possible, taking care to retry on any error.
Projects like nullmailer (http://untroubled.org/nullmailer/) work well, but they are somewhat limited (in the nullmailer case having multiple instances running on the system requires patching).
Spoolgore tries to address the problem in the easiest possible way.
Why Go ?
MTAs (and similar) tend to spawn an additional process for each SMTP transaction. This is good (and holy) for privileges separation, but spoolgore is meant to be run by each user (or single application stack) so we do not need this kind of isolation in SMTP transactions.
Go (thanks to goroutines) allows us to enqueue hundreds of SMTP transactions at the cost of few KB of memory.
Obviously we could have written it in python/gevent or perl/coro::anyevent or whatever non-blocking coroutine/based technology you like, but we choose go as we wanted to give it a try (yes, no other reasons)
implement rate limiter (lot of services limit the number of emails you can enqueue per minute/hour)
windows is not supported, if you want to use Spoolgore on it, just patch the code to not use syscall.SIGURG and syscall.SIGTSTP