sendmail-tryqueue is a script that tries sending an email by reading it
on the standard input and forwarding it to the standard input of a given
sendmail command. If the command fails, the email and command are stored
on disk, in a "queue directory".
Emails stored in such manner can subsequently be re-sent by the script, which makes it a good addition to a MUA’s configuration.
Similarly to a sendmail command, sendmail-tryqueue reads an email on its
standard input, and forwards it to the standard input of the sendmail command
it’s passed as argument.
But it’s not intended to be a drop-in replacement with an additional side-effect, so it doesn’t implement any of the flags generally implemented from the sendmail API.
Example:
# Try to send an email
$ cat test.eml | sendmail-tryqueue send sendmail -- e@ma.il
# List the emails that could not be sent, and were consequently queued
$ sendmail-tryqueue list
# Try to re-send the emails in the queue,
# in the same order they were originally sent
$ sendmail-tryqueue flushQueues are stored in ${XDG_DATA_HOME}/sendmail-tryqueue/queues by
default.
The emails are written as .eml files, and sendmail commands as .sh
scripts. Their name starts with a millisecond-based timestamp, contains
the subject of the email, and is seeded with a random number in case the
same email was being sent at the exact same time.
A generic database was purposefully not chosen to handle storing and reading the queues to allow interacting with the data stored more easily (e.g. with UNIX tools).