Unwanted Horizontal Lines in Outlook: Causes and Possible Solutions
What are these horizontal lines in Outlook and why do they appear?
In certain Outlook versions (on Windows), 1px-high “Ghost Lines” can appear in mailings, especially on colored backgrounds. This happens because Outlook relies on the Word rendering engine when displaying emails, which can mistakenly create these lines. They seem to appear randomly: sometimes in different places and sometimes only after opening the email multiple times.
Which Outlook versions are affected?
All Windows versions of Outlook, including Outlook 365, that use the Word rendering engine. This effect usually does not occur on other email clients or webmail services.
How exactly does the problem manifest itself?
Is there a solution or a workaround?
A 100% solution does not currently exist. However, there are approaches that reduce the probability of these lines appearing (e.g., adjusting the HTML code). Some of these workarounds may cause undesirable side effects, such as layout shifts. Microsoft is aware of the issue, but no official fix has been released so far.