Problems sending emails

I’m using a custom domain.

I need email for only two purposes: The confirmation email and password reset.

After over 100 tests in three days, less that 10% got through immediately. Most have been deferred by the receiving domain and sent on sometimes hours later, some never appeared on SendGrid’s activity screen at all.

My latest attempt was a test of my first use of a sendgrid legacy template with an unsubscribe group. It has been deferred 7 times so far, and still not processed. (Just received it, 37 minutes after sending.)

This has happened to two different yahoo email addresses, and one private server (large school, 1000+ users) using outlook.

I don’t think it’s a SendGrid issue. It’s the receiving email domain refusing it.

Do you have an insights on the cause and on a fix?