Yeah, these problems are consistent with Microsoft email. I've been using Sendgrid to send emails for our SaaS and some customers who use Microsoft email don't

Yeah, these problems are consistent with Microsoft email. I've been using Sendgrid to send emails for our SaaS and some customers who use Microsoft email don't receive transactional messages at all. There's nothing you can do about it. Also, seen this happen with other apps too. Transactional messages don't arrive on Microsoft email addresses.