What’s a message ID and how is it generated?

ESPs handle authentication by providing the DNS records you need to publish and configuring their sending infrastructure to align with those records. For SPF, the ESP gives you their include statement to add to your domain's SPF record, authorizing their servers to send on your behalf. For DKIM, they generate or let you upload key pairs and tell you which DNS records to create for the public keys.

DMARC alignment is where ESP authentication gets more complex. The ESP must send email in a way that passes either SPF or DKIM alignment with your domain. This typically requires custom sending domains rather than shared ESP domains. Higher-tier ESP plans often include dedicated sending domains and DKIM signing with your own domain to ensure DMARC passes cleanly.

Setup complexity varies significantly between ESPs. Some provide clear wizards that walk you through each DNS record with verification tools. Others require more technical knowledge to configure properly. Larger ESPs offer managed authentication services where they handle domain configuration entirely. Before choosing an ESP, understand their authentication options and ensure they support the authentication level your domain policy requires. Proper authentication is non-negotiable for deliverability; make sure your ESP makes it achievable for your technical capacity.