DKIM is a technology that looks at the sender’s domain (after the @ sign) and verifies if the email really did come from that domain. If it doesn’t match then there's a possibility that the email won’t go through or will get flagged as spam.
On a deeper level, a digital signature is attached to each email sent. When the email reaches the receiving person, DKIM checks the signature by looking at two things in the email header (which normally most recipients don’t see): a private key, which is the signature of the sender and a public key, which verifies the hidden private key.
VerticalResponse uses a variety of digital sending techniques, including DKIM, to make sure your email gets to the inbox of your email recipients.