When you configure your MachForm to use SMTP server, all the forms within your MachForm will use that SMTP server.
I assume you're using one instance of MachForm for all your 50 different domains?
If you're using separate instance of MachForm for 50 different domains, then you'll need to go through each MachForm instance and configure them with SendGrid SMTP server.
That's all what you need to do.
It doesn't matter if you're using WordPress or any other CMS, since MachForm is working independently.
Also, white labelling in SendGrid is not requirement for MachForm itself but it does increase the delivery of your emails. I suggest to contact SendGrid tech support directly regarding white labelling process.