Were the emails normally being sent manually through some provider, but now going from a mail program on the server?

If so, you might consider using SendGrid or something like that with their API. https://sendgrid.com/docs/API_Reference/Web_API_v3/Mail/inde...

If not using a service, would be interested to hear what configuration (DKIM or whatever) was necessary to add to your existing mail setup to get it to work.

Although I am just totally speculating on what you were talking about.