Add existing user email + from address

Hi,

we have an issue with the emails coming from our wordpress multisite install. Adding new users is fine, the from address is: wordpress@ourdomain.com. However when an existing user is added to a site, the from address is reverting to the underlying AWS hostname: wordpress@ec2-123-456-789-12.compute-1.amazonaws.com

Can you advise on why the difference? And what we can do about it?

Additionally we'd also like to modify the "add existing user" email template - there is no setting for this in the Network Admin dashboard. Ideally we don't want to have to edit the wordpress core - can this be done via filter hooks or similar?

Many thanks,
Paul