From the documentation:

<select name="recipients[]" multiple="multiple">
<option value="jackATTmysite.com">Jack</option>
<option value="maryATThersite.com">Mary</option>
</select>

Allows the user to select one or more recipients.

First question: Does "recipients" need to be defined as hidden prior to the selection box as in:

<input type="hidden" name="recipients" value="xyz"/>

Second question is if the value can be derived from the formmail.ini file. For example:

<select name="recipients[]" multiple="multiple">
<option value="abc">Jack</option>
<option value="def">Mary</option>
</select>

where the formmail.ini file is:

[email_addresses]
abc=jack@hissite.com
def=mary@hersite.com