If you're going to use your domain (or a custom domain) with an email platform, try, if at all possible, to use your main domain name.
Here's why I say that.
Let's pretend I'm a large multi-national corporation called Al Iverson. I own aliverson.com. Nobody else can (successfully) send mail as aliverson.com. Maybe I'll send mail as aliverson.com, or perhaps I'll configure a subdomain, email.aliverson.com, to use with my new partner email platform. Using my main aliverson.com domain name (or a sub-domain of it) is best, because from a consumer's perspective, only I can send mail as aliverson.com. Other people can try, but it'll deliver poorly because of a lack of email authentication (and a strict DMARC policy can help to encourage that poor delivery).
Why don't I register aliverson-email.com and just use that to send email instead? That way I can have separate domains and don't have to worry about configuring DNS for multiple providers. The reason I don't consider that a best practice is a simple one -- anybody could have registered aliverson-email.com. Or email-aliverson.com. Or aliversonmarketing.com. Or one of a thousand other variations. How does a recipient for sure know, at a glance, that mail is truly from me, Al Iverson?
Well, the best way they know, is they see the from address domain is aliverson.com. Not some other domain. It's that simple. Savvy email nerds might look at the email authentication and see that the email passes DKIM and SPF checks, giving fairly solid proof that the mail in question was indeed authorized by aliverson.com. When those email nerds look at the same things for aliverson-email.com, they see that the email passes DKIM and SPF checks, but not with that same rock-solid connection to aliverson.com. And then if you check the MX or other DNS records for aliverson-email.com, they typically point at the email platform or service provider, not the core company. And there's no MX record for aliverson-email.com pointing back to aliverson.com. All together, there's no clear visible technical connection in the email headers that proves that aliverson-email.com is connected to aliverson.com.
Sometimes there are very legitimate reasons as to why you can't use your core domain for your email platform. People still do that today and it can be unavoidable. For example, perhaps they don't have the technical expertise to manage the DNS entries necessary for multiple service providers. If you have no choice, then yes, use aliverson-email.com, and make sure email authenticated properly, and that you maintain a good sending reputation, and that you link directly back to www.aliverson.com (only) to minimize consumer confusion. Do it if you have to, but don't do it if you don't have to.
But the BEST way to do it, if you have the technical skill or if you can lean on a consultant or partner to help you support it, is to use your main domain name, or a subdomain of your main domain name.
Laura Atkins posted a cheeky-yet-not-inaccurate take on this topic over on Word to the Wise earlier this year. That's not exactly how I feel about it, but it does sort of sum up how ISPs feel about cousin domains. They find it confusing and occasionally frustrating.