Sending solicited mass email

Question

Our company does work environment surveys, and these surveys are filled in online. All participants are sent a link to their survey in an email (personal code included).

Answer 1

I'd suggest that getting your clients to whitelist your server would be the most appropriate state, after all you are in a contractual relationship with them so some configuration as a result would be reasonable.

The validity of the email address list that you are sent must lie with the client, particularly if they aren't pulling it from their live email address list. There's little you can do, or should be expected to do, to validate those addresses if the client doesn't provide the appropriate responses.

Also, this isn't a programming question even remotely (okay, if you throttled your emails to a common domain you've more chance of getting it through but that's nothing to do with the programming, more about the nature and operation of SMTP and the protection systems therein). Questions like this are more suited to SuperUser.