SRD75 on "[Plugin: WP Mail SMTP] Emails have lost their formatting."

When WP-mail-smtp is activated, emails sent through the website lose their HTML formatting.

If I deactivate WP-mail-smtp, emails are sent with HTML formatting okay.

This behaviour occurred about 2 weeks ago, but no plugin changes occurred during that time.

Advice appreciated.

https://wordpress.org/plugins/wp-mail-smtp/