I happen to be running a test with them this week to see what deliverability looks like on their end. So far only 79% of their messages have been accepted for delivery by remote service providers. These are non-spam, non-transactional, typical business correspondence messages.

In their activity feed right now, SendGrid traffic is being blocked by comcast, GoDaddy, iCloud, and a cornucopia of smaller services.

Here's one of SendGrid's outbound IPs, listed on multiple RBLs used by other services: https://mxtoolbox.com/SuperTool.aspx?action=blacklist%3a167....

On the receiving end, I've had to deal with waves of spam from SendGrid for years, and it's especially difficult because they carry a mix of spam and legitimate traffic, so blocking them causes complaints and not blocking them also causes complaints.