Messages sent to and from CompanyName may be monitored to ensure compliance with internal policies and to protect our business. Emails are not secure and cannot be guaranteed to be error free. Anyone who communicates with us by email is taken to accept these risks.
In my opinion this is perfect definition of whole mailsystem based at pop3 and smtp protocol. It is doddored like whole Internet (which "be or not to be" is based at 13 global nameservers). The main problem is that is has been invented in time - when security was not considered.
Of course - we have the tls/ssl in mail comunication, but we cannot enforce the second server to use it, because of compatibility, and risk of loosing some mails.
No provider can guarantee email delivery. ISP's have different rules about SPAM detection based on content, subject and how many of their users are reporting emails as SPAM. The best way to make sure that your specific survey invitation is not blocked as SPAM is to pre-test it with free email addresses from hotmail, yahoo etc. But as I said WITH NO GUARANTEE :)
next problem is that we cannot be sure, that person who send us an e-mail is the real person, not an robot or sth. Yes - there is an SPF - but enforcing it like tls/ssl can make some serious problems to delivery. And like you can read here SPF in his simplicyty is vulnerable too... which is not a good news.