I’m having this issue on siteground.
I applied the patch and emails are sent again. Thanks @marius.
@sweetmelody. This is what I did on one site. Siteground use cPanel
1. Go to File manager
2. Open the folder wp-includes
3. Edit the pluggable.php file
4. Find line 352
5. Change it FROM $phpmailer->setFrom( $from_email, $from_name );
TO $phpmailer->setFrom( $from_email, $from_name, false );
6. Save the file.
Although it goes without saying, I’ll say it anyway. Backup the site first.
The same can be done using ftp, which I did on all the other sites affected.
Hope it works for you, @sweetmelody.
Once again, thanks @marius.