I am the Network Administrator at my company, and we use constant contact for employee news updates.
We have about 250 employees, and all but 2 recieve emails from CCSEND on a regular bases. Everytime, these two individuals do not get the email. So far, this is the troubleshooting taken:
I am starting to believe the problem is with the API they use now, or a problem on the constant contact end.. Is there any troubleshooting left that I need to attempt before escalating?
This sounds like a pretty clear case of the contacts' server receiving a message from Constant Contact and dropping the message without sending the originator (us) any bounceback/undeliverable/blocked message information. It is industry standard to send a 'bounceback' message to the originating sender, but if their server isn't doing that, there isn't too much that we can do on our end other than have their administrators check their server logs to see why our message didn't make it into their mailbox.
We'll probably need to have you contact our phone or email support (firstname.lastname@example.org) to get you to our compliance department, which handles deliverability issues, but before I do, let's rule out an API issue. I don't see any reason that an API your customer is using could affect them receiving our messages, but do you know something I don't? Our API, if they are using it, merely bypasses the UI (allows automation of importing contacts, sending emails from Constant Contact, etc), but the underlying mechanisms (the servers we use to send emails, etc) for those things are the same whether you are using our API or not.
API Support Specialist