The neckbeard within me was satisfied when I managed to whisk off a patch set via the command line. It took a little time to set it up and the documentation was scarce, so here's how I got it done.
I installed msmtp and ca-certificates with:
$ sudo apt install ca-certificates msmtp-mta
Then set up my ~/.msmtprc:
defaults account riseup host mail.riseup.net from email@example.com user lukewm auth on port 587 tls on tls_trust_file /etc/ssl/certs/ca-certificates.crt account default : riseup
Then, to validate everything was working, I let loose the wondrous magic of SMTP:
$ echo "Hi!" | msmtp --debug firstname.lastname@example.org
Finally, I was able to send a Darcs patches with:
$ darcs send --mail --to email@example.com
Darcs politely asked me what to send and that was that.