DKIM stands for DomainKeys Identified Mail and it's basically a way to prove that emails from your domain are actually from you and haven't been tampered with. Think of it like a digital signature that gets added to your emails. When you send an email, DKIM adds an encrypted signature to the header, and when it arrives at your friend's inbox, their email server checks that signature against a public key stored in your domain's DNS records. If it matches, the email is legit. If it doesn't match or is missing, that's probably why your emails are going to spam or getting rejected.
The DKIM selector is just a name that points to the specific DKIM key in your DNS records, because you can have multiple DKIM keys for different purposes. Common selectors are things like "default," "mail," "k1," or sometimes your email provider's name like "google" or "sendgrid." To find your DKIM selector, log into your hosting control panel or cPanel and look for email settings or DNS records. If you're using a third-party email service like Google Workspace, Office 365, or an SMTP service, they usually tell you what selector to use in their setup documentation. You can also check your email headers from a sent email to see what selector is being used, or just ask your hosting support what your DKIM selector is since they set it up. Once you have it, those DKIM checker tools can tell you if it's configured correctly or what's broken.
