We are a Cloud SaaS provider (www.yourcircuit.com) and we use email as part of our sign-up process. We are using IBM Softlayer and SendGrid as our email sender. We have taken several steps to prevent our emails being caught by our customers' SPAM filters, such as signing emails with DKIM, including Unsubscribe Links, sending multipart emails (HTML and Plain text).

But still we get reports from customers using Microsoft 365 email, that they do not receive our emails.

What steps can we take to either get our domain whitelisted by default, or, are there any other email formatting steps we can take to prevent the emails being classified as SPAM?