Security in Post Office
DKIM (Domain Keys Identified Mail) is a technology for identifying a sender's authenticity by using a digital signature linked to the domain's name. Your electronic signature confirms that a message was not intercepted and changed before being sent from the sender's mail server.
A digital signature is designed to determine the identity of an e-mail and help fight spam and phishing. Messages receive a digital signature on the sender's mail server. The sender cannot set a signature his/herself unless they are the administrator of the server from which the message is sent.
For more information, read this article about DKIM.
Mailing security policy
A SPF-record (Sender Policy Framework) is a system for preventing mass spam mailings that confirms the authenticity of the messages' sender by checking their IP address. A domain's SPF-record allows it to precisely determine the list of servers from which messages are allowed to be sent on behalf of the domain.
Configuring the SPF-record is not mandatory to work with mail on your domain. However, it may help you avoid losing sent messages. You must be able to edit the domain's DNS records to configure the SPF-record.
For more information, read this article about SPF.