There may be times you need to paste content from another source, like Microsoft Word, into one of your messages. When doing so, keep in mind that what you see in one program may be different that how it will display in your message because of the likely inline formatting you'll be bringing along with it.
Typically, it is not recommended that you copy formatted text from another platform and paste it into an email message in the Message Editor because other platforms introduce inline styles and other things that may not be compatible with email messages. This unseen formatting can also negatively impact your deliverability.
If you must copy content from an external source, paste it into the Message Editor using the Paste from Word option or the Paste as plain text option next to it.
This will strip out most of the extraneous code. But even these tools are not impervious; there are still some issues that you can run into, like invisible characters and hidden HTML elements that aren't visible until the message lands in your inbox or Deliverability Testing.