Post notification email format problem

I’ve recently changed the theme of our website from Hueman to TwentyFifteen & installed Styles plus the Styles TwentyFifteen add-on.

I’ve not consciously changed the post notification emails but there is now a problem with them. These emails have left border in the email content area which seems to be over-writing much of the post notification text.

Making the the window displaying the email larger or smaller moves this border. In a small window, the border disappears & the whole contents of the email can be seen.

The emails print correctly and the links in the email work.