I see that Let's Encrypt is recommending Certbot as the Acme client to use. I have a question about Acme clients in general though. We need a SAN certificate for an Exchange Server. Is it possible to create the certificate from a different computer with the Acme client installed or does it need to be done from the server itself? I do not have access to the server, so I am just trying to determine whether this is something I will even be able to do myself and then provide them the certificate, once it is created.
In principle, yes, this is possible. How you'd go about it could vary, and it'd likely be easier if you were to use the DNS challenge rather than the HTTP challenge. For this to be a sustainable solution, though, you'd need a way to automate deploying the cert from the system that obtains it to the system that would use it.
Note that Certbot in Windows is no longer supported or recommended; you should use a suitable Windows client instead.
Usually the easiest solution will be to have software running on the server itself, just because then it's usually easiest to automatically deploy to where it needs to be. For Windows servers, the most-recommended around here are Certify the Web for the GUI fan, simple-acme for the command-line fan, or Posh-ACME for the Powershell fan.