From a technical perspective, there's no challenge in sending emails from @not-companyA.com if your domain is actually @companyA.com, so long as you own @not-companyA.com and can set up SPF and DKIM on them.
I've run into this issue before, and I think what you're trying to do makes sense—a better solution I've seen, though, is to set up your Marketo mass-emails to be coming from a subdomain of your main URL. The main problem with sending from @not-companyA.com is that it looks like spam/phishing. For example, if I were to receive an email from an address claiming to be Bank of America, but it was sent by an address with @marketo-bankofamerica.com, I would immediately report it as phishing. Same with any other company.
If you're sending from a subdomain of your main domain, you get the deliverability protection, as it's treated as a separate domain, while maintaining the branding power. You can set up SPF and DKIM on subdomains as well. Just make sure your "reply to" goes to an email address that someone actually watches, such as your main domain.
Regarding domain tracking ability when sent from another email domain, there is no negative impact because these are two siloed functionalities within Marketo. There are three moving siloes that are relevant here:
- Email domain you're "sending from" in Marketo—you could make this anything you want so long as it's set up with SPF and DKIM in Marketo (conversationally, you can also send it from any other domain, if you're okay doing that without SPF and DKIM in place).
- Email link branding and shortening—this tracks clicks and opens based on people hitting these links. Each lead/contact gets a different unique URL that tags them with a cookie upon clicking. You can see what these are when you send an email to yourself and hover over the link. Because this is a redirecting link sitting in the middle, tracking clicks work by people hitting these redirecting links. Regardless of whether these point to google.com or to your companyA.com, they'll track link clicks.
- Munchkin tracking on your site—this is the JS snippet on your site which watches for web page visits. This happens independently, so in the same way that you got a visit from a web referral of linkedin.com, you can see you got a referral from your email link branding.
Edward Unthank | Founder, Etumos