I'm not a customer but one of our employees communicates with two vendors that use your service and hasn't been able to receive product lists via e-mail generated by your company. We were instructed to whitelist the domain r20.rs6.net but upon further research we discovered this breach reported:
Glanced through your forums and didn't see anything addressing it. Don't feel comfortable whitelisting you if this hasn't be addressed at this time.
Hello @BrianS76050 ,
We are aware that the account credentials of one of our customers was compromised, and has already been addressed. To date we have not found evidence of a breach of Constant Contact’s environment.
If you need a full list of CTCT domains to safelist