If you’re not on a dedicated IP plan with sendgrid this is common. Happens with dedicated still if you cooked your sending reputation.
Sendgrid is the bottom of the barrel when it comes to providers. They’re notorious for not delivering to certain providers. Hotmail or outlook most commonly.
Postmark, mailgun, Amazon SES are the best.
My all in one email plug-in and email builder provides everything you need to succeed with emailing in bubble
