If yes - the .cda file isn't the actual track, it's a technical reference only and not of any use. You need to use a digital audio extractor to copy the track(s) off the CD and onto your hard drive (in one format or another). Then attach to the email as with any other file.
Extracted CD Audio files in their natural state (.wav) are huge - about 10mb per minute. You may therefore prefer to convert to (or extract as) MP3 - which can go as low as 1mb per minute approx with little loss of quality.
If by cda you meant something else, sorry I''ve wasted your time.
When you burned the CD-RW - did you choose (in your burner software) to make it as an 'Audio CD'? If yes, then my original comment is still valid (makes no difference whether an audio CD is commercial or homemade, nor on CD-R or CD-RW).
Unless you still have the original files on your hard drive, you need to extract (using a method called DAE - digital audio extraction) the relevant track off the CD back onto your hard disk.
As I said before, you can't use (for example) explorer to copy the cda file - it isn't of any use (it's more like a 'shortcut' than a real file).
You need to work out which software (if any) you have that allows you to do DAE. Depending on the software, some will only do direct DAE which creates a .wav file; some will convert to MP3 on the fly. If the former, you'll want to convert to MP3 separately, using suitable software.
You have a CD-writer - yes? This must have come with some burning software - Roxio Easy Cd or Ahead Nero most likely. As far as I know, both of these (Nero certainly) include DAE functions and MP3 converters.
If you have two drives - a CD/DVD reader AND a CD and/or DVD writer - you may find that only the WRITER is equippede for DAE - so put your CD in that drive.
Without knowing what software you have, I can't be more specific, but in summary, the process is:
Put CD in writer drive
Use DAE software to extract the track onto the HDD. Depending on software and what you select, it may come out as either WAV or MP3
(If necessary): Convert WAV to MP3.
Attach MP3 to email and send.