Sage 100 emailing uses SMTP settings configured in Company Maintenance.  You get the credentials from whoever manages your outgoing email server / service, and add / test the settings in Company Maintenance. 

If you're running a much older version, some of the newer authentication options won't be there, so you need to find something that works with what you have, or upgrade.

You would setup Sendgrid to use API Keys. You would then use this in Sage. The User login would always be "apikey" and the password would be the api key you created. Outgoing server is "smtp.sendgrid.net" and the port is 587. SMTP Encryption is "TLS/STARTTLS". You should be able to set the domain to your domain.

When you create the key on Sendgrid you need at least the Mail Send rights (you don't have to give full access)