where to set sendmail_from

Would it be ok to add the ini_set(…) line in wp-config.php?

You can’t add anything to the wp_config.php file other than those described in https://codex.www.ads-software.com/Editing_wp-config.php

What is it you are trying to do via email?

Just a notification e-mail triggered by actions selected in the Better Notifications plugin. Right now, the e-mail works ok if I send it to my personal e-mail ([email protected]), but not the company e-mail. What I was told is that the e-mail is being sent without any ‘From:’ info in the header, and this causes the e-mail to be blocked from the network entirely.

I would suggest that you contact the author of the Better Notifications plugin about this. You can post to the plugin’s dedicated forum here via https://www.ads-software.com/support/plugin/bnfw

Thanks, just did that now.