When you are composing a new mail in your Outlook and pasting a text by copying it from a html page or word doc you will have to re-format the text by changing the font colour, font size etc to make it match to the one that you have already.
For example here is the new mail which has the default fonts set, when I copy some text from a web page and paste here it looks like this,
Instead of changing this text to the one that you have already. You can simply select the text that you want to and click Format Painter button (shown in the screen shot below) and select the newly pasted text. Then your newly pasted text will get the text format that you expect (shown below)