Rotating DKIM keys involves generating a new key pair, publishing the new public key in your DNS, and updating your email system to sign emails with the new private key. Do this periodically and maintain a secure process for generating, storing, and deploying keys to ensure that old keys are retired safely without disrupting email delivery.

Using weak cryptographic algorithms in DKIM signatures can make them susceptible to forgery and compromise, undermining the integrity and authenticity of your emails. It's important to use strong, recommended algorithms (like RSA with a key size of at least 2048 bits) to maintain robust email security and protect against attacks.

Longer DKIM signatures, resulting from stronger cryptographic algorithms and key lengths, offer better security but can slightly increase the size of the email. While this generally has a negligible impact on deliverability, ensuring that your email infrastructure can handle these signatures without issue is crucial for maintaining both security and deliverability.

While DKIM significantly enhances email authenticity and integrity by verifying the sender and ensuring content hasn't been tampered with, it doesn't address all aspects of email security, such as the legitimacy of the sender's domain. Combining DKIM with SPF and DMARC provides a comprehensive approach to email authentication and security.

When using third-party vendors to send emails, you'll need to add their DKIM keys to your domain's DNS records or have the vendor use your DKIM keys to sign emails. Coordination with each vendor is crucial to ensure that all emails are properly authenticated and aligned with your email security policies.

For managing DKIM records across multiple domains, maintain a centralized inventory of domains and associated DKIM keys, use consistent naming conventions for selectors, and monitor the performance and security of each domain's DKIM setup. Regularly review and update keys, and ensure all domains adhere to current best practices for DKIM implementation.

Content filtering and anti-spam technologies may examine DKIM signatures as part of their evaluation process, using the presence of a valid DKIM signature as one factor in determining an email's legitimacy. However, DKIM alone is not a guarantee against filtering; the content and reputation of the sender also play critical roles.

Tools like DKIMValidator, Mail-Tester, and various online DKIM check tools can be used to test and validate DKIM signatures before full deployment. These tools simulate the email sending process and check for proper DKIM signature creation, helping identify any configuration issues.

High-volume email senders should ensure their DKIM setup can handle the load without causing delays, use dedicated IP addresses for sending to improve reputation management, and monitor their DKIM performance closely. Implementing DKIM signing at the edge of the network (e.g., at the SMTP gateway) can also help manage the signing load more efficiently.

Implementing DKIM in cloud-based email services can involve challenges like limited access to DNS for DKIM record management, dependency on the service provider for key management and rotation, and potential issues with key synchronization across distributed systems. Overcoming these challenges requires close coordination with the service provider and careful planning to ensure a seamless DKIM implementation.