Sometimes, new members (especially those new to forums) may not always be aware they need to validate their account. I used to have a simple message back on 2.x.x to help them and have now done something similar for IP.Board 3.
It's a very simple skin edit to create a box with a reminder message in it.
ACP > Look / Feel > Manage Skin Sets / Templates > (choose skin) > Manage Templates / CSS > Global Templates > GlobalTemplate >
Scroll down about 3/4 of the way down that template and find:
<div id='content' class='clear'> <a id='j_content'></a> <if test="mainpageContent:|:$html">{$html}</if> <br class='clear' /> </div>
Add this above, after the closing div on a new line:
<!-- validating message--><if test="authenticating:|:$this->memberData['member_group_id'] == $this->settings['auth_group']"><div align='center'><br /><div class='general_box' style='width: 75%;'><h3><img src='{$this->settings['img_url']}/comment_new.png'> Thank you for registering!</h3><div align='center'><br /><strong>As part of the registration process, we require you to validate your email address. You will need to do this before you can post.</strong><br /><br />Please check your inbox for your validation email from us and click the link provided in it. If you did not receive this, please <a href="{parse url="app=core&module=global&section=register&do=reval" base="public"}" title='{$this->lang->words['resend_val']}'><strong>Click Here</a></strong> to resend it.<br /><br /></div></div></div><br /></if><!-- end of validating message -->
Save the template.
This is the simple message all validating members will now see, along with the extra link to click for a new email if required. You can easily customise the message (for example by allowing validating members to post in a certain forum) as required.