Offering an "Unsubscribe" mechanism
Problem
You want to offer a method for respondents to unsubscribe from future invitation e-mails.
Solution
Add an "unsubscribe" calculation to the questionnaire and a link to it in invitation e-mail messages.
Discussion
It might be counter-intuitive, but offering a way for potential respondents to exclude themselves from invitation e-mail messages is actually a good thing, for a number of reasons. Offering a way to unsubscribe:
Offering an unsubscribe mechanism is a two-step process. First, create a calculation and an information screen for this purpose in the questionnaire. Make sure that this section of the questionnaire is isolated from the rest of the questionnaire so that it is not reached in the normal course of filling out the questionnaire. For example:
The calculation can be anything that suits your needs. The solution above adds "_UNSUB" to the e-mail address so that the address is no longer deliverable. This has the advantage of keeping the address information for future reference (i.e., "email@domain.com" becomes "email@domain.com_UNSUB" but the original e-mail address is still accessible in this field) and of requiring no other intervention to avoid further mailings to this respondent.
Second, insert a link in the invitation messages to trigger that portion of the questionnaire. In the example above, the link could look like this:
This link opens the case referenced by the current _telkey, in project PROJECT, at question UNSUB, and displays the unsubscribe confirmation message.
A URL to the unsubscribe feature in the questionnaire can be supplied in the "list unsubscribe" option of cwemail.cgi and in the Autoemail pound instruction. Some Web mail system recognize this instruction and display a special button using the URL. A default value for "list unsubscribe" can be defined in the "list unsubscribe" pound instruction.