SendGrid just like any other email service provider is very strict because they operate in an industry that is highly abused by spammers and scammers. They have to deal with the risk of a user who may cause a problem that will result in their IP being restricted by email systems like Gmail or Outlook.
In most cases, your SendGrid account will be quickly suspended if you have billing issues, have not configured email authentication properly, are sending prohibited content, and have a high email bounce rate or spam complaint.
Sendgrid will also not provide you with details of the policy you violated. Therefore, the best thing to do is to consider alternatives.