When you copy some text in an app (for example Notes or Clean Text) and then paste it on Mail, the size and color of the pasted text could be different than the one you already write in the message (for example "Hi Mary,"). This is because Mail could use a different size and color for text that you type directly and the pasted text.
To solve this problem you can use Clean Text for iOS to write and compose all the email message, and then paste it on Mail. In this way all the message text size and color are uniform.