There is a known Outlook bug where certain combinations of font size and font family will trigger a rendering error that’s creates a 1px horizontal line of the email background color to appear. This bug appears most often at the bottom of the table in which the text is contained, but sometimes will appear in other places in the template.  In some cases, this line will disappear once the email is sent.  This bug will also sometimes appear depending what version of Outlook your recipients are using. 

Further reading and discussion can be found here:

https://litmus.com/community/discussions/4990-outlook-2016-1px-horizontal-lines-showing-up-in-the-body

https://www.emailonacid.com/blog/article/email-development/how-do-i-get-rid-of-the-lines-in-outlook-emails/

This Outlook bug is an extremely frustrating issue and PoliteMail's HTML team has developed a few workarounds you can try to resolve this issue.  Bear in mind, adjusting the table tools and HTML may break the table structure of your template.  Before you attempt to do anything SAVE YOUR TEMPLATE!

1.  Save and then reload the template.  Sometimes just closing the template will fix the issue.

2.  Send the template to yourself using PreFlight Test Send.  Sometimes the line will disappear once the email is sent.  If the line does not appear in your PreFlight Test Send, it should be fine to send out.

3.  Adjust the size of your font.  The bug appears less when your font size is an even number rather than an odd number.  So if you have a font size of 13, change it to 14.

4. The background color of the container table which holds the image table (a box inside the box) is supposed to be applied to, or cascaded (CSS cascading style sheets) down from the container to those inside tables, but doesn’t always.  In the toolbar, go to Options > Page Color.  Change this color to the color of the problem section.  This will mask the line and it will be invisible to your recipients.

5.  You can try and add an additional small line in the HTML code.  Often times, this will force the table down, covering up the line break.  You will need a HTML editor like Dreamweaver in order to perform this action.  In the code, find the outer table data <td> of the table before the line.  Add either of the following code:

<p style="line-height: 2px; font-size: 2px; margin: 0;">&nbsp;</p>

<p style="margin:0;"><span style="font-size:0; line-height: 0">&nbsp;</span></p>

If none of these solutions work, contact the PoliteMail Support Team by clicking Request Support.