To create a MD5 only Opt-Out list you will need to do the following:
1) Open the Opt-Out List's settings
2) Scroll down to the section called Storage Settings
3) Enable MD5 Only
4) Then save the changes by selecting Update List
IMPORTANT: changing a list which is in plain text, with data already stored in it, will switch the entire file over to MD5 and this is NON-REVERSIBLE. If you wish to keep the data already in plain text, and add the MD5 list also, we recommend setting up an additional list for the MD5 file, and then creating a Group List combining the two for distribution to affiliates. Any Group list with an MD5 component to it will ONLY be able to be distributed in MD5.