If we have to vouch on 1 factor that can drastically affect your delivery rates, that would be
It helps because of 2 main snow-balling effects. But before we get to that, let’s try to understand what Custom domain entails.
Custom Domains help you
Keep all the links in your email matching your ‘from-email’s domain/sub-domain
The unsubscribe, web version links show your-domain.com
Custom domains or custom tracking is a strategy perfected by cold emailing softwares because they have to worry the most about landing in the inbox, now this practice is also common for email marketing tools.
Custom Domains for Authenticity
Email service like Gmail & outlook generally use SPF, DKIM to evaluate the authenticity of the emails & the sending domain. But to distinguish if it is a one-to-one email or a bulk email, they look at a multitude of factors, one of which is the similarity between sender domain & links in the email.
Custom Domains for User Experience
Recipients are better able to recognize you, resulting in lower complaint rates. When users view & open the links in the emails, they know whose email they are clicking on. If they don’t see the domain of the sender in the URL, they can see something is fishy.