We've recently begin working with a dedicated IP address and are in the process of warming. Yesterday we send a small email blast to some franchisees (about 200), and a husband and wife both bounced back with:
The duo are the only two who have come back to this send with this error - our franchisees are all authorized to use servers unique to their markets, so we are not system administrators for them. I've checked in our settings in Marketo and our domain is fully verified, so I'm fairly sure the error is nothing on our end, though if I'm wrong, someone please set me straight!
From previous experience, I believe the issue that this is bouncing back is a problem with our domain blocked on their end, however we've had no problems with previous email communication with them (both from our mail client and from other services like MailChimp). Specifically, I think the error is that they are blocking the actual from sender, which is the President over their region.
We're still looking to get buy in from the system on Marketo, so I wanted to see if anyone had previous experience with going back to recipients on this error.
Unless you're using a branded MAIL FROM (a.k.a. SMTP envelope sender) then the envelope sender is @ *.mktomail.com.
So they have you on a local blacklist... that's their prerogative! (That's assuming the error is not purposely misleading... nothing obligates a recipient to give you the details behind a 5xx if they think you may use the info to circumvent their anti-abuse setup.)