Because freemail domains (eg. those of your personal email accounts, such as Gmail, Outlook.com, Yahoo, etc.) weren't meant for email marketing! They're perfect for getting in touch with friends and family, but using them for bulk email sending can be your one-way ticket to spamville.
Why? Because spammers love using freemail domains, which means ISPs will give you the evil eye if they see you using one of those. This in turn might damage your deliverability right from the get-go.
To be on the safe side, you should use a registered domain (eg. firstname.lastname@example.org or your company's ) as your sender, which will also benefit from manual authentication and an E-goi CNAME for the best possible deliverability. You can use it in E-goi right away - as long as it hasn't been registered too recently.