Make sure your mail gets delivered with full authentication by enabling DKIM with your third party sending services. You need DKIM signing on their side before the DKIM keys you store in Authenticate will be of use. Also, in many cases third party services will give a DKIM key that you enter into Authenticate for signing to work.
Turn on DKIM signing outside of Authenticate
Start with your vendor's help documentation, searching for "DKIM". Most vendors have very helpful guides to enabling DKIM signing. If you can't find one, reach out by opening a ticket, sending an email, or giving them a call. Usually, you can get DKIM signing turned on with no hassle. It is a security feature that's standard in the market and prevents a critical vector of cyber attack.
The vendor will need to generate a key. This can be done directly, by the sending service administrator, from the vendor’s dashboard, or by the vendor’s support team. Once the key (or keys) have been generated, you can add DKIM keys to Authenticate in the detail page about this sender. Look for DKIM in the Configuration section.
After the keys have been published, the keys may need to be activated or validated from the vendor’s side before they start signing the emails.