I can't use a Gmail or Yahoo address to send my emails
Yahoo doesn't allow sending emails from servers other than its own. (They use an email authentication mechanism called DMARC)
So if you try to send emails from systeme.io or any other software with your Yahoo address, your emails will be rejected by the receiving mail servers and won’t reach their destination.
As for Gmail, it's not recommended to send emails to your list from a Gmail address because you may have problems with delivery.
Case of Microsoft (Outlook, MSN, Hotmail, Live)
It is also not suitable to use "outlook", "MSN", "Hotmail" and "live" email addresses for delivery reasons.
These are not business email addresses, so the inbox servers will tend to reject emails from these domains.
For all these reasons we do not allow systeme.io users to use Yahoo, Gmail, and Microsoft (Outlook, MSN, Hotmail, live) addresses to send emails.
This is in the interest of all users to ensure better email delivery.
We recommend that you register your domain name with a mailbox host of your choice, it only costs a few dollars per year.
Please check out this article to learn more about email deliverability.