HTML Mail SMTP settings not recognised

After updating from webform 7.x-3.18 to 7.x-4.0-rc3, upon submitting any webforms, the following message was encountered: "Unable to send e-mail. Contact the site administrator if the problem persists." My site is currently running Drupal 7.

Mail modules setup being used as follows:
HTML Mail
Mail System
SMTP Authentication
Webform

A new HTML Mail module class, HTMLMailSystem__SmtpMailSystem, was created at Mail System settings, however, webform did not appear to recognise this setting and emails were still being sent through the default mail system.

With reference to New MIMEMail SMTP settings not recognized by Webform, the following line was added to line 4394 of the webform.module file:

$systems[] = 'HTMLMailSystem__SmtpMailSystem';