Linux – “send-mail: Applet not found” when using mail command on alpine linux

You may use sendmail or "sendmail look alike" provided by postfix/exim/... .

/usr/sbin/sendmail -i -- $recipients < message_file

-i - do not treat lines with leading dot specially

You may use more exotic "sendmail look alike" (e.g. provided by msmtp) to send directly via another smtp host without "system wide" configuration.
msmtp is distributed in debian so it is likely to be included in other linux distributions.

https://packages.debian.org/stretch/msmtp

Package: msmtp (1.6.6-1)
light SMTP client with support for server profiles

msmtp is an SMTP client that can be used to send mails from Mutt and probably other MUAs (mail user agents). It forwards mails to an SMTP server (for example at a free mail provider), which takes care of the final delivery. Using profiles, it can be easily configured to use different SMTP servers with different configurations, which makes it ideal for mobile clients.

To get the picture to be displayed in-line, it must be encoded as a valid MIME object with a Content-Disposition: inline header in it.

The mpack command can do this.

Try sending an email like this:

mpack -s "TEST mail" -c image/jpeg download.JPEG <MASKED>@example.com

Or if you want to output the result into a file instead of sending it directly:

mpack -s "TEST mail" -c image/jpeg download.JPEG -o email-with-image.txt

Then you can send it later with e.g.:

mailx <MASKED>@example.com < email-with-image.txt

If you want to add text to your message before the image, write it into a file, and add a -d text-before.txt option to the mpack command. For adding text after the image, just appending it to the mpack-produced file should work.