Embed images or link to external images in emails in legitimate mass mailings

Question

It turns out to be quite easy to send emails with .NET that use embedded images. By embedding I mean actually include the image as an MIME attachment.

I'm just trying to figure out whether or not I should embed images as resources for mass mailings (to opt-in / existing customers). Alternatively I would just reference images with a <img src>

Reasons for embedding images

• Spam filters may be less likely to block emails - because no tracking pixels exist
• Email clients may be more likely to show the images - because no tracking pixels exist
• Available for offline viewing
• We don't need to host the images indefinitely

Reasons for not embedding images (IMG src attribute pointing to an external site)

• Spam filters may be more likely to block large files if we have lots of images
• We get to host the images and change them if we made a mistake
• We can track views of emails in server logs
• Blasting out a tonne of emails should take a lot less time
• We take less of a server bandwidth hit for the hosted images555
• Sending out many emails with embedded images may take a long time becuse we have to send 400kb for each email

I'm sure there are more reasons.

I'm most concerned about spam related issues. Curious for anybody's input

Answer 1

You can at least check the spam-score of the email you've created. Checking two versions with and without embedded images. A lot of mailing applications (mass-mailers) offer some sort of spam check functionality. Or you could use a tool like: http://spamcheck.postmarkapp.com/