We have run into an issue recently where our emails are being blocked or refused by Trend Micro. Trend Micro has an email reputation score for different IP addresses and if an IP Address is on their QIL list then the IP address is flagged and emails sent from that IP Address do not go through. It's so bad that our own servers won't accept email campaigns for our own email newsletters. I would highly suggest that someone regularly check each Constant Contact IP address with the Trend Micro reputation manager: https://www.ers.trendmicro.com/reputations to ensure that all of the IP addresses are clean and not being marked as spam.