There are useful tools out there that can help you figure out where to send a spam report to. I use the ARIN Regional Internet Registry and abuse.net nearly every day to look up spam reporting (abuse) contacts for IP addresses and domains. Some folks use SpamCop (which historically does not play nice with ESPs, so it's not as valuable to me). I don't necessarily have the time or skills to build something as technically complex or useful as these tools, but I did want to try to make it easier for people to find spam/abuse contact information for various sending email platforms (ESPs, email service providers). To that end, I've reached out to various providers and asked them to share contact info so that I can share it with you here.
If you get an unsolicited or otherwise unwanted email message, and if the domain in the from header, return-path header, message-ID header, DKIM d= header, or the domain name of the IP address matches any of the domains listed below, you'll be able to see which ESP platform is likely responsible for the message is and where to send that spam report. I can't personally vouch for the policy compliance efforts of every single entity listed here, but I do know that many of them DO take spam reports seriously and WILL take action to help stop spam and guide clients on how to do the right thing.
My hope is that by listing them here, people searching for information on these domains in Google will more easily find out where they should send spam complaints.
If you'd like me to add your platform to this list, I'd be happy to do so! I didn't leave certain companies out on purpose -- I figure I had to start somewhere and that it would grow over time. Contact me to have your information added.
Email Service Provider Abuse Contact List
ESP or Platform: Constant Contact
Report spam/abuse to: firstname.lastname@example.org
Domains: *.constantcontact.com, *.confirmedcc.com, *.ccsend.com, *.rs6.net
ESP or Platform: Cordial
Report spam/abuse to: email@example.com
Domains: cordial.com, cordial.io, sp.crdl.io
ESP or Platform: Klaviyo
Report spam/abuse to: firstname.lastname@example.org
Domains: *.klaviyomail.com, *.klaviyomsv.com, klclick.com, klclick1.com, klclick2.com, klclick3.com, klclick4.com, klclick5.com, klcdn.com, klcdn1.com, klcdn2.com, klcdn3.com, klcdn4.com, klcdn5.com, klcdnd.com, klv1.io, klv2.io, klv3.io, klv4.io, klv5.io, klv6.io, klv7.io, klv8.io, klv9.io, klv10.io, kmail-lists.com, kmail-manage.com, kmail-subscribe.com, myklpages.com
ESP or Platform: Listrak
Report spam/abuse to: email@example.com, firstname.lastname@example.org
Domains: listrak.com, lstrk.net, esp1.co
ESP or Platform: Mailkit
Report spam/abuse to: email@example.com
Domains: mkt-synd.com, mkt-ags.com, txnmail.com
ESP or Platform: Mapp Digital
Report spam/abuse to: firstname.lastname@example.org
Domains: bluehornet.com, mapp.com, shortest-route.com, muc.domeus.com, ec-messenger.com, ec-cluster.com, cncluster.net, uscluster.com
ESP or Platform: Marketo Engage, part of Adobe Experience Cloud
Report spam/abuse to: email@example.com, firstname.lastname@example.org
Domains: mktomail.com, mktdns.com, mktroute.com, mktosender.com, marketo.com
ESP or Platform: Postmark
Report spam/abuse to: email@example.com
Domains: postmarkapp.com, wildbit.com, mtasv.net, dmarcdigests.com
ESP or Platform: Salesforce Marketing Cloud/ExactTarget/Pardot
Report spam/abuse to: firstname.lastname@example.org
Domains: *.exacttarget.com, *.exct.net, mktg.salesforce.com, pardot.com, pd25.com
ESP or Platform: Selligent
Report spam/abuse to: email@example.com
Domains: *.emsecure.net, *.slgnt.eu, *.slgnt.us, *.slgnt.de, selligent.com, gridinbound.blob.core.windows.net, gridinboundus.blob.core.windows.net
ESP or Platform: SendGrid
Report spam/abuse to: firstname.lastname@example.org
Domains: sengrid.net, sendgrid.com, sendgrid.info and/or IP addresses in AS11377
ESP or Platform: SharpSpring
Report spam/abuse to: email@example.com
Domains: shspma.com, marketingautomation.services
ESP or Platform: Unica
Report spam/abuse to: firstname.lastname@example.org
Domains: unicadeliver.com, unicadeliver.net, unica.com
Note: Some ESPs have the option for clients to use their own domain name in various places in the email headers or message body. Thus, an email message you received could have been originated from an ESP noted above even if it doesn't mention one of the domain names above. It can't hurt to check any other domain names against abuse.net to see if abuse.net has mapped a particular domain to a particular ESP (and thus a particular spam reporting contact).
Last updated: July 2, 2020.