BTW - I wouldn't use SendGrid for anything. The amount of SPAM that company is sending at the moment is ridiculous. They just ignore SPAM complaints. 

Dulouz:

 

BTW - I wouldn't use SendGrid for anything. The amount of SPAM that company is sending at the moment is ridiculous. They just ignore SPAM complaints. 

 

FYI spam is not an acronym, so it shouldn't be used in all-caps.  The people behind the canned version object.

To answer the OP's question, a free smtp mail relay service will be blackholed for spam purposes. People abuse free things.
Depending on the settings proferred by your MSP you could use your ISP's own SMTP (without authentication as a requirement) or you need an authenticated SMTP service.  SPF and other similar controls usually want you to use authenticated SMTP from the same place as your email is hosted, but it's not always absolutely necessary... however it is highly recommended.