It's likely being caused by the compression and stretching of the document to upload it and import it. In all honesty, if you're going to just have a plain-text letter as your email, I'd advise either:
Copying and pasting the text into a text block directly within the email. You can still have the signature of the letter be inserted as an image after the sincerely and before the typed-out name.
Or link to the letter as a document, rather than have it display as an image in your email.