<fieldset>
    <legend>Enter contact email</legend>

    <label for="primary">Primary address:</label>
    <input type="email" placeholder="drone@globex.com"
           pattern=".+@globex.com" size="30" required
           title="Must be a globex.com email address" />

    <label for="secondary">Secondary addresses:</label>
    <input type="email" placeholder="foo@this.com, bar@that.com"
           size="35" multiple
           title="Zero or more addresses, separated with ','" />

</fieldset>

Output