This message only applies to users who don't use their SMTP server for sending emails.
Because of the upcoming policy changes (mandatory DKIM and SPF verification) of our current email provider (Mailgun.com) we won't be able to send emails on your behalf unless you switch to the new provider Postmarkapp.com by verifying your email addresses in My account / Email & domain authentication. Or alternatively you can set up your SMTP server in My account / SMTP server.
You can verify all email addresses by clicking the verification links in confirmation emails from Postmark, and Postmark will allow us to send emails on your behalf.
However it's higly recommended to also authenticate your domain by adding SPF and DKIM records to your DNS (there's a guide in My account / Email & domain authentication) to increase the deliverability of your emails. It's a little bit more complicated, but it's definitely worth it, since Postmark will be explicitly authorized to send emails on your behalf. They will less likely end up flagged as spam.