Hi MSMS,
Google as last I tested will use the complete EHLO/HELO identity issued by the sending server during the SMTP transaction as defined in RFC7208 section 2.4 ( https://tools.ietf.org/html/rfc7208#section-2.4). This behaviour cannot be changed. This does mean that in those instance, best practice would be to publish an SPF record for the domain name associated with the mail server when possible and where alignment is achieved.
I hope this helps.
AM
