Have a file of your users' name and email address laying around that you need to shoot an email to? I did and didn't want to leverage my server's SENDMAIL, etc so I just use mailgun..

send_emails.sh

#!/bin/bash
#
# Changed a few things up from my original version I am using
# but thought others might want to have an easy way to send
# emails to users using mailgun via bash..
# 
# be sure to replace the FROM field, subject, content, etc
# just read the script and follow-along and modify accordingly.
#
# the contents of the file I am reading are email username:
# someone@something.tld an-user
# another@there.tld another
# where@there.tld user3

filename="$1"
apikey="api:key-1234abcd5678efghijklmnop" #your mailgun api key
domain="shat.io"

while read -a line
do
    email=${line[0]}
    username=${line[1]}
    curl -s --user "${apikey}" \
    https://api.mailgun.net/v3/${domain}/messages \
    -F from='shat <shat@shat.io>' \
    -F to="${username} <${email}>" \
    -F subject='Put a real subject here homie..' \
    -F text='Hello there,
    
    Shat here, showing you how to send some emails using mailgun.
    
    Feel free to use this example!
    
    Kind regards,
    
    Shat' > /dev/null
    echo "queued email to user: ${username} at ${email}"
    sleep 0.5 # I throttle, just because.
done < "$filename"

chmod +x or execute:
$ bash send_emails.sh userlist.list