This is usually a result of copy-pasting from an external source. When you copy-paste into Pipedrive's mail client, it will keep the formatting of the outside source in which you originally composed it, such as a word processor or other email client.
To get around this, you can use Email template feature to compose templates for emails you send regularly. Alternatively, when you do need to copy-paste something, make sure to paste any text without formatting (CTRL-SHIFT-V on Windows, CMD-SHIFT-V on Mac).
Note: The Email Sync feature is one of the many useful features of Pipedrive’s Advanced, Professional, and Enterprise plans. To learn how to switch plans, click here.