DMARC (Domain-based Message Authentication, Reporting & Conformance) is the latest and greatest advance in email authentication. It means that your emails from your gmail email addresses ( firstname.lastname@example.org ) can only be sent via GMail Servers and not via servers of any other email service providers ( like Mailchimp, SendX etc.)
For more technical and detailed understanding of DMARC kindly refer here and here
Changes by GMail
Spammers can sometimes forge the "From" address on email messages so the spam appears to come from a user in your domain. To help prevent this sort of abuse GMail is incorporating DMARC which gives domain owners more control over what Gmail does with spam email messages from their domain.
So going forward it means that if you are sending emails from email@example.com via SendX then they will very likely be either marked as spam or get bounced. Other providers like Yahoo, AOL etc. already follow DMARC policy.
How does it affect you and solution
Very likely it doesn't. Since most of you are already sending emails from firstname.lastname@example.org on SendX. This change only affects users who were using email@example.com as from address to send emails via SendX.
How to fix it
Going forward you will have to create a email account using your domain (yourdomain.com) and use it in your SendX from email address. This will ensure that any emails sent to your contacts won't be bounced or marked as spam by gmail.