How do I send emails from a different domain?

I have been given the task of figuring out how to send emails from a different domain. As an example, lets say my company's domain is 'abc.com'. I'm looking to send emails on our clients behalf. Let's say the client's domain is 'xyz.com'. The IT staff at xyz.com are ready and able to help however they can. We have a lot of clients, so I imagine the steps might be different for different clients, depending on the what technologies they use.

What steps do we need to take to send emails on our client's behalf? What steps do our clients need to take to allow our emails to send correctly.

There seems to be DNS, SPF, and DKIM issues.

From what I can tell, no one on the internet has created a comprehensive guide for this.

Might someone be able to assist?

Generally speaking, you can't. Spammers would really love for this to be possible, and this is the reason why there are lots of systems in place to avoid this.

If you have a legitimate reason for sending emails on behalf of someone else, there are two solutions:

• Send the emails through their mail server.
• Ask them to configure their anti-spam settings (SPF, DKIM, DMARC, whatever) to allow your mail server to send emails using their SMTP domain in the sender address.

Both solutions require their willing cooperation, but the technical details depend on which mail server and anti-spam settings they are using.