I hope this can be helpful for someone else with the same problem as I had.
I noticed on our servers that mailparse was only installed for cli and fpm and not for apache2, thus I had to include it for Apache too as follows:
we are using apache2 with php7.0. on ubuntu 16.04
1. find path of file mailparse.ini:
# cd /; find / -name *mailparse.ini
/etc/php/7.0/mods-available/mailparse.ini
/etc/php/7.0/fpm/conf.d/20-mailparse.ini
/etc/php/7.0/cli/conf.d/20-mailparse.ini
2. I then copy mailparse.ini from cli to apache2 and needed to edit and include the full path:
# cp /etc/php/7.0/cli/conf.d/20-mailparse.ini /etc/php/7.0/apache2/conf.d/20-mailparse.ini
# nano /etc/php/7.0/apache2/conf.d/20-mailparse.ini
and add full path as: extension=/usr/lib/php/20151012/mailparse.so
the full path I got from running # find / -name *mailparse.so
3. Restart Apache:
# systemctl restart apache2
4. See if apache restarted without errors:
# systemctl status apache2
and now my test.php file with phpinfo(); shows mailparse support! :-)