The use of DKIM (DomainKeys Identified Mail) usually improves deliverability.

The establishment of DKIM for a sender domain provides a digital signature in the header of all emails that are sent with the sender's domain on promio.mail. With this signature the recipient can validate the identity of the sender, and thus verify the authenticity of the e-mail sender.

For sender domains of promio.mail setting up DKIM is mandatory.