Almost every email message has a "from" address. This is the email address the email appears to come from, and which is displayed above the message body in the user agent. If you hit the reply button, this is also the address to which the reply is going to be sent (unless the mail also contains a "reply-to" header).
We intentionally say it is the address the message "appears" to come from. If you send mail, you can use every possible email address as the from address, even addresses that do not even belong to you. This freedom can be used for harmless practical jokes ("see, you just got an email from the president"), but can also be abused. As a domain owner, you do not want others to send emails using your domain name!
DKIM can be used to prevent this. It allows receiving parties to verify whether an email was indeed sent by you and that the content of the email (including the attachments) was not modified during transport.
This signing technology is based on the mechanism of related private and public keys. As a domain owner, you are the only one who possesses the private key and you are the only one who can sign emails. Your public key is published in a DNS record of your domain, so that everyone can check whether a signature really came from you. This technology ensures that mails can only be signed by you, and that they can be verified by everyone else (because everyone has access to the public key to check the signatures). This makes it impossible for spammers, phishers, or anyone else to use your domain as from address and sign mails out of your name, simply because they do not have access to your private key.
Signing of messages
SMTPeter can sign your mails with DKIM. In order to do this SMTPeter needs to know which from addresses you use with SMTPeter. You can configure something called sender domains via SMTPeter's dashboard. If you create a sender domain, SMTPeter creates DKIM keys and informs you how to update the DNS records. This is a one time procedure. Once a sender domain is configured, SMTPeter automatically signs mails with from addresses identical to the sender domain.
Do you already have private and public key pairs, and do you want SMTPeter to use these? No problem, you can use the dashboard to install your own private keys too. It is also possible to let SMTPeter know that it should always add a signature of a certain key, even if the from address of the sent mail is different from the sender domain.
Mind you, even if you have your own keys, it is in general still a good idea to leave the signing of email to SMTPeter. SMTPeter normally also modifies your email (for example to track clicks and opens, or to inlinize CSS code) and this invalidates signatures that were added before.
You should of course not be sending out mails with different "from" addresses than your sender domains. However, if you happen to send out mails with a different from address SMTPeter will see if it can use one of your sender domain keys and still fulfill all necessary requirements.
Automatic DKIM key rotation
The private keys are stored on the SMTPeter servers and are never exposed. The technology behind the public and private keys is very secure, yet, if someone spends a lot of time on it, keys can be broken. Therefore, you want to generate new keys every now and then. If you use SMTPeter's standard suggestions, it will rotate your keys automatically. If you want to use SMTPeter with your own generated keys, updating the keys is your own responsibility.