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 by sending an email invitation to different free email addresses from hotmail, yahoo, gmail, etc.
While QuestionBuilder makes every effort to make sure that our IP Addresses are whitelisted and not blocked, we cannot guarantee delivery of emails. It is the survey administrators responsibility to make sure that emails do not get blocked by the recepient's mail servers.
