Hi @haet. I didn’t get notice you had replied otherwise I would have replied sooner.
The issue affects all email whether sent via a plugin or not.
The reason I mentioned POST SMTP was to demonstrate that email headers and WordPress inherent email issues were not affecting the sites in question because POST SMTP resolves them, which means the bug is most likely somewhere within WP HTML Mail.
The plugins that send their own mail in my two affected sites are:
1) Matador Jobs
2) ACF Advanced Forms (which uses Advanced Custom Fields to build email forms)
As I say, all mail is affected by the bug including regular WordPress emails.
I use WP HTML Mail in only these two sites.
Thanks for your help, Hannes.