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When you are sending out a survey through the survey mailer, in the upper right hand corner is some blue text that says "Advanced Options" If you click on this you can get more options for distributing your survey. 

*Email Expiration - You can set a number of days by which the email will expire.  The number of days ranges from 3-90 days.
*Test Email - If you want to see what the email and survey will look like before you send it out to your entire panel, you can choose an email address to send a test email to.
*Link Settings - You can select out of three options:  individual link, multiple completes per link, and anonymous link.  The individual link is the default setting for the Survey Mailer.  If you use this, it is not necessary to protect your survey with a password.  The individual link only works once per email.  The multiple completes per link means that the survey can be taken more than once from the same link.  When you use the anonymous link you will not be able to track which respondent said what.  When you send an anonymous link, it will usually say that the survey is from noreply@qualtrics.com.