Hello,
I have more or less the same issue here.
Since approx 2 days, all emails coming from Contact Form are showing 'Wordpress' as the sender AND contact form is sending from a wordpress@mydomain.com address.
Since this address does not exist it creates a mess in my contact/support software (Zendesk).
Zendesk receives emails from wordPress@mydomain.com and tries to send an automated response.
Since the address does not exist Zendesk receives a mailer daemon notification and generates a suspended ticket....
Please note this issue occured very recently, was there an update?