After going slightly insane and trying to style every element but still not be able to get the horizontal line in Outlook 2016 away. After googling and not finding a solution just other people with the same issue.
I started to remember that adding in a blank space can solve the issue in some dodgy clients and in this case it did!
If you are adding in a button into a template and see horizontal line at the bottom, try to add in a
just before the table cell ends.
Example:
<table>
<tr>
<td>
<div><!--[if mso]>
<v:roundrect xmlns:v="urn:schemas-microsoft-com:vml" xmlns:w="urn:schemas-microsoft-com:office:word" href="http://" style="height:40px;v-text-anchor:middle;width:200px;" arcsize="10%" strokecolor="#1e3650" fillcolor="#556270">
<w:anchorlock/>
<center style="color:#ffffff;font-family:sans-serif;font-size:13px;font-weight:bold;">Show me the button!</center>
</v:roundrect>
<![endif]--><a href="http://"
style="background-color:#556270;border:1px solid #1e3650;border-radius:4px;color:#ffffff;display:inline-block;font-family:sans-serif;font-size:13px;font-weight:bold;line-height:40px;text-align:center;text-decoration:none;width:200px;-webkit-text-size-adjust:none;mso-hide:all;">Show me the button!</a></div>
</td>
</tr>
</table>
