Send email with curl over SMTP from a shell script

# curl is already installed; alternative: sudo apt install msmtp

# message.txt
From: Your App <hello@yourdomain.com>
To: customer@example.com
Subject: Backup finished

The nightly backup completed at $(date).

# send (STARTTLS on 587)
curl --ssl-reqd --url "smtp://smtp.queensmtp.com:587" \
  --user "$SMTP_USER:$SMTP_PASS" \
  --mail-from "hello@yourdomain.com" \
  --mail-rcpt "customer@example.com" \
  --upload-file message.txt

# implicit TLS on 465
curl --url "smtps://smtp.queensmtp.com:465" --user "$SMTP_USER:$SMTP_PASS" \
  --mail-from hello@yourdomain.com --mail-rcpt customer@example.com --upload-file message.txt

# msmtp for cron/mail(1) users — ~/.msmtprc (chmod 600)
defaults
auth on
tls on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
account default
host smtp.queensmtp.com
port 587
user your-smtp-username
password your-smtp-password
from hello@yourdomain.com
# then:  echo "body" | msmtp -s "subject" customer@example.com