How to configure exim with docker for sendmail
According to the documentation, Joomla has native support for sending mail to an SMTP server. However, if that won't work (e.g., if you have extensions that ignore Joomla's settings and always try to use a local sendmail binary), you can install a sendmail alternative that performs the necessary SMTP transaction for you.
A common solution is msmtp, which is often used by mutt users to provide SMTP server for the mutt mailer. The msmtp client is command-line compatible, so you can install msmtp and then:
ln -s /usr/bin/msmtp /usr/sbin/sendmailProvided that you put the appropriate configuration in place, this should solve your problem.
Just about all major MTAs support a "sendmail compatibility mode", and Exim does support that. The fix is simple. As root, do:
cd /usr/sbinln -s exim sendmailThen when your application call /usr/sbin/sendmail and passes a message to it on STDIN, exim will parse the commandline options in a sendmail compatibility mode.