MailChimp may not sound like a legit source of traffic, but neither did MailRU when it first came on the scene. That's not to say MailChimp isn't monkey'n around, just that it probably needs further investigation.
Don't know anything PegTech them except they're a server farm and bad behavior has come from their ranges enough times for me to ban them. So far these are the PegTech ranges I have:
184.108.40.206 - 220.127.116.11
18.104.22.168 - 22.214.171.124
126.96.36.199 - 188.8.131.52
184.108.40.206 - 220.127.116.11
18.104.22.168 - 22.214.171.124
126.96.36.199 - 188.8.131.52