I could have sworn I'd reported this previously, but I can't find the issue. Anyway, as you can see from the screenshot, there is an underscore between the issue icon and issue type description. I am using ThunderBird email client.

In file summary-topleft.vm, see lines 48-49

## This is all one line so that we don't get the bug in IE6 where the whitespace gets an underline
<a href="${baseurl}/browse/${issue.getKey()}">#renderIcon(${issueType.getIconUrl()} ${issueType.getNameTranslation($i18n)})</a>

I'll update this ticket with more info...