We have found that USPS has recently had more scrutiny around the type of mail that is being sent with each postage type, and they are being more strict on what can and cannot be sent via each class. Here is decision tree guidance for your reference when choosing between Marketing Class mail and First-Class mail:
The enforcement of these USPS Marketing Mail Eligibility standards are unfortunately out of Lob’s control, have been in effect since June 1, 2005, and are enforced at the discretion of the USPS Customer Support Rulings agent. Please note that standard mail should not be utilized for sending mail that includes personal information or sensitive data. If you are planning any marketing mail campaigns that include the personal information of the recipient, please send that mail first class to avoid USPS rejection and resulting delays in mail delivery time.