phpmailer_init ($phpmailer->AltBody)

Forgive me if I’m being a complete muppet. I’m having problems with emails not being delivered to some email addresses. Using the wp_mail() function I have set a 'Content-Type: text/html; charset=UTF-8' header so all emails are sent as a HTML version only.
Running a test at https://www.mail-tester.com highlights that I should also send a plain/text version of the email. As my emails are complex, I hit upon the idea of hooking into phpmailer_init to send a default plain/text email using the $phpmailer->AltBody function. This all works well without using Post SMTP Mailer but as it’s really great is there a way of getting this working or do I just need to rethink the whole thing?

Thanks
Steve Harper