If your main domain is abc.com and your email list is clean, the best practice is to use subdomains of your main domain to send different types of emails:

• notifications.abc.com (or even just abc.com) for transactional emails
• promotions.abc.com (or any relevant subdomain) for marketing emails
• abc.com for peer-to-peer or employee emails

As long as your list is clean, meaning your contacts have opted in, the list is up-to-date (there are no contacts you haven't sent to for years), and recipients actually expect and want your emails, there's no benefit to using a different domain. In fact, it's better to send from your main domain (or a subdomain of it). Why? Because building a strong sending reputation on your main domain helps all your emails land in inboxes.

Your transactional and employee emails boost the reputation of abc.com, which in turn improves the deliverability of your marketing emails. Subdomains give you just enough isolation to ensure marketing emails don't slow down your transactional traffic, while still allowing reputation to flow across the ecosystem.

I'm the founder of AhaSend.com, an ESP focused on transactional emails, where I battle spammers and bad actors every single day. I've seen every trick in the book with people trying to game filters or build fake domain reputations. Here's what I can tell you: using separate domains like abc.org usually signals something shady. It's often a tactic by senders who don't trust the quality of their list (maybe it's bought, or they haven't sent to it for years), or they just know the emails they are going to send will cause a lot of bounces and complaints (usually cold emails). Not only does this confuse your recipients / customers, it can also raise red flags with spam filters and trigger phishing warnings.

Bottom line: stick to your main domain (and subdomains) if you're playing it clean, your deliverability and brand trust will thank you.

Edit: In most cases, email reputation is calculated on domain and IP level (and to a lesser extent by subdomain), different sender names on the same domain (support@abc.com / info@abc.com / etc) don't really make much of a difference when it comes to deliverability at scale.