For those who aren't aware of the changes with Gmail and Yahoo coming this year...

There are five major requirements slated to occur in connection with Gmail and Yahoo Mail's new rules: 

1. One-click unsubscribe (RFC 8058) via the list-unsubscribe header is required for all commercial/promotional emails.
2. There are now published spam complaint rate guidelines.
3. Unsubscribes must be processed within two days.
4. Senders must have valid email authentication. Any bulk (one-to-many) sender must have valid Sender Policy Framework (SPF), Domain Keys Identified Mail (DKIM), and Domain-Based Message Authentication, Reporting, and Conformance (DMARC) records.
5. Senders must have valid PTR (FCrDNS) records