My guess is most QuickBooks users, don't understand what's going on, nor want to, unless they have an IT department behind them that understands how email servers and blocklists work. As invoices aren't delivered and payments aren't made, I wouldn't doubt it'll continue to bubble up. More people will search for this issue, more people will post they have the same problem. But it'll take time.

When I posted this originally, the first date we saw this behavior was March 30th 2022. Prior to that, no issue. So we're about a month and some change into the 3 sendgrid IP's being listed on a blocklist.

If I had to venture a guess, they either started to use sendgrid for emailing outbound from their service. Or sendgrid switched their IPs since they aren't documented anywhere on the Intuit website at this point. My workaround for the time being, is to allow those IPs through, but hold all attachments for review. It's obnoxious, but at least it continues to protects us, and allow us to receive safe emails, until Intuit does something productive about it.

I'm debating on going a step further to send a notice on each email back to the sender stating that the emails are coming from malicious IP addresses, in the hopes that each one opens a ticket with Intuit. My hesitancy is that they'll just create a mail rule to delete those responses since people generally take the easy way out of everything.