You could create a metakeys in case you want to persist the email address data. On the public template you could have Label that shows "Email address" and anchor control that gets the data from the metakey.
For instance you could add the following in the public template
"mailto:email@example.com?subject=<asp:Literal runat='server' Text='<%# ((CmsContentBase)Container.DataItem).GetMetaData("
CustomValue").ToString().Replace("&", "%26")%>' />
You could use the same expression for the mail as for the subject if you store the email as metakey
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