DKIM

Your email provider signs each message with a private key, and publishes the matching public key in your DNS so receiving servers can check it. If the signature matches, the receiver knows the message came from your domain and wasn't changed on the way.

Example: you move your email to a new provider. It gives you one or more DKIM records to add to your DNS. Until you add them, your messages go out without your domain's signature, and some may land in spam.

Missing or wrong DKIM is a common reason business email lands in spam after a move. Each service that sends for you, such as a newsletter tool, usually needs its own DKIM record.

DKIM works together with SPF and DMARC.

What it means for your price: DKIM is free to set up and comes with business email services. What it buys you is deliverability. When comparing providers, check whether they set up DKIM for you, and make sure it's part of any email move.

How to check yours: send an email to a mailbox you have elsewhere and look in the message headers for 'dkim=pass'.