I ran into an issue with Constant Contact that basically banned one of my mailing segments of around 500 emails. The rest of the list was okay to send to, just one segment was blocked. The issue was a false positive spam test by Constant Contact and they refused to unlock that segment. Even had they unblocked it, the CS experience was still bad enough to raise a warning flag.

I've since moved on to MailChimp and have been much happier with it than I was CC.

I'm glad to see them improving on an already highly functional free-tier.