[6logics]

I fixed it by changing the code in function bp_send_email in file /buddypress/bp-core/bp-core-functions.php.

Before:

$must_use_wpmail = apply_filters( 'bp_email_use_wp_mail', $wp_html_emails || ! $is_default_wpmail );

	if ( $must_use_wpmail ) {
		$to = $email->get( 'to' );

		return wp_mail(
			array_shift( $to )->get_address(),
			$email->get( 'subject', 'replace-tokens' ),
			$email->get( 'content_plaintext', 'replace-tokens' )
		);
	}

After:

$must_use_wpmail = apply_filters( 'bp_email_use_wp_mail', $wp_html_emails || ! $is_default_wpmail );

	if ( $must_use_wpmail ) {
		$to = $email->get( 'to' );

		return wp_mail(
			array_shift( $to )->get_address(),
			$email->get( 'subject', 'replace-tokens' ),
			$email->get_template( 'add-content' )
		);
	}

[6logics]

we do need to send rich HTML emails. Is there a way we can configure BuddyPress’ custom instance of PHP Mailer to use our SMTP server?

I am also looking forward to this option. I am using Gmail SMTP plugin to send email and it is causing to send plain text.