Definition
DKIM
A cryptographic signature added to each outgoing email so the receiver can check it came from your domain and was not altered.
DKIM (DomainKeys Identified Mail) lets a sending domain sign its messages. Your mail server adds a signature header computed from the message content with a private key; the matching public key is published in your DNS. The receiving server fetches the key, verifies the signature, and knows two things: the message really passed through a server authorised by your domain, and the signed parts were not changed on the way.
For cold-email senders DKIM is not optional. Major providers expect authenticated mail from anyone sending in volume, and a message that fails or lacks DKIM starts at a disadvantage before anyone reads a word. DKIM also protects the reputation you build: without a signature, anyone could send mail claiming to be from your domain, and their behaviour would be counted against you. DKIM is one of three records that work together, with SPF stating which servers may send and DMARC telling receivers what to do when checks fail.
In Seegnals the Deliverability tab shows SPF, DKIM and DMARC status for each sending domain, checked hourly. A failed lookup is shown as unknown, never as missing, so a temporary DNS hiccup does not send you chasing a problem that is not there.