I don't know or care about spam, but SendGrid is one of the most prolific sources of malware and phishing emails out there. Their opaquely rewritten URLs are an annoyance to security teams everywhere, enough that I've seen enterprises simply block ct.sendgrid.net or any URL with /ls/click, which isn't the kind of thing large companies typically have the will to do.

I work with phishing emails, I love sendgrid, your sandbox sucks if you are not detonating URLs to see where they lead to and rewriting means as soon as sendgrid finds out about an account sending spam/phishing they can disable all those links which isn't doable when they are doing the same thing with multiple domains, urls, subject lines, attachment names,etc... leaving you with live phishing links you don't know about in someone's inbox.

Any modern paid email protection service will also work nicely with sendgrid and the like.

You worked at orgs that didn't have the best decision making if they are blocking sendgrid. Talk about throwing the baby with the bathwater. Lazy mitigations made bad security. This is especially true here since sendgrid is hardly the only one of their kind so you will play whackamole blocking all kinds of legit services because... you don't have toolimg that can follow 301's??

What does your phishing email filtering pipeline look like?