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We are experiencing troubles with an important provider that makes all our emails bounce. We tried a lot of solutions to no avail, now we would like to contact the problematic ESP in question (ig.com.br)

A quick research on mxtoolbox makes it appear like they are using the services of "locaweb.com.br" :

mx:ig.com.br Find Problems    mx  
Pref    Hostname    IP Address
10  mx1.ig.correio.biz  177.153.23.241
Brazil BR
Locaweb Serviços de Internet S/A (AS27715)

However, locaweb denies being involved. As I don't understand very well how mxtools work, I'm not able to contradict them.

So can I confidently say on the base of this report that they indeed are in charge of dispatching the emails for ig.com.br?

My objective is just to contact anyone in the relevant organization, but I seem to have trouble even at step 0: determining what is the relevant organization

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Keep in mind that this does not mean that even though an ISP controls an IP, does not automatically make them responsible for the server that IP points to. In this case, the IP that the MX records for ig.com.br are pointing to are definitely owned by them. Here's the DNS information for the MX records:

$ dig mx ig.com.br +short
10 mx1.ig.correio.biz.
20 mx2.ig.correio.biz.
20 mx3.ig.correio.biz.
20 mx4.ig.correio.biz.

Those domains point to the following IPs:

$ dig mx1.ig.correio.biz +short
177.153.23.241
$ dig mx2.ig.correio.biz +short
186.202.4.42
$ dig mx3.ig.correio.biz +short
187.45.217.20
$ dig mx4.ig.correio.biz +short
200.234.204.130

And those IPs are controlled by the following:

$ whois 177.153.23.241 | grep owner:
owner:       Locaweb Servi?os de Internet S/A
$ whois 186.202.4.42 | grep owner:
owner:       Locaweb Servi?os de Internet S/A
$ whois 187.45.217.20 | grep owner:
owner:       Locaweb Servi?os de Internet S/A
$ whois 200.234.204.130 | grep owner:
owner:       Locaweb Servi?os de Internet S/A

Locaweb may or may not control the server or servers that these IP addresses point to, but they are most certainly in control of those IPs, and where those IPs point to. I hope that helps!

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    Just because an IP block is owned by an ISP does not mean that they are responsible. ISPs often reallocate IPs to customers. The OPs question has the answer. Just look at the MX record. – closetnoc Apr 12 '17 at 16:04
  • @closetnoc Indeed, but as the ISP controls what happens with the IP, they should hopefully at least be able to assist with letting the OP know who they should be talking to. I'll edit my answer to make that more clear at the top of the answer, rather than at the bottom. – FCTW Apr 12 '17 at 16:07
  • No. Not really. I was one of the first registered ISPs and allocated IP addresses and IP address blocks. It is not the ISPs business after that short of abuse and I am sure they do not want to get involved in another companies business. The larger the ISP, the less they involved they can be. That is reality. Like I said, the OP is ignoring the evidence he has provided. The answer is there. – closetnoc Apr 12 '17 at 16:18

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