Recently I did some testing based on complaints about failed email deliverability to some standard free email accounts (Windows Live Hotmail, AOL, Gmail, and Yahoo).
The Tests Using a basic HTML message based on a basic template, I ran some tests.
All the emails had the same innocuous subject line and were HTML messages, since it is well established that plain text email messages are highly deliverable.
None of the email accounts had the sender address added to the address book to avoid the junk folder.
Message 1 HTML, no images, no links.
Message 2 HTML, web images, no links Message 3 HTML, embedded images, no links Message 4 HTML, no images, with links Message 5 HTML, web images, with links Message 6 HTML, embedded images, with links The Results All the messages were delivered to the inbox of all email accounts successfully except in the case of embedded images.
The messages with embedded images were redirected to the junk folder in Yahoo's mail client.
Styles In Yahoo, Windows Live Hotmail, and AOL the background color and other styles were overridden by the email client and did not display correctly.
This did affect the appearance of the message where light colored links are offset by the dark background color.
This occurred for all the messages to those accounts since I did not change the basic layout or style, just the images and links.
Embedded Images In the embedded image tests, with and without links all the message were delivered to the inbox, some of the clients would display the embedded images when approved by the viewer.
Yahoo put the embedded image message in the junk folder.
Web Images In AOL and Gmail clients, web based images triggered a prompt for the user to allow the images to be displayed.
Yahoo and Hotmail displayed web images automatically with no problems.
Links The links I used were click through and unsubscribe links using an internal IP address and a non-standard port, specifically port 81.
These types of links are usually suspect since some spam filters do not like IP address or alternate ports.
AOL in particular used to check each link in the message as it comes in.
However, none of the accounts balked at allowing an email message to the inbox with a click through link and an unsubscribe link with these elements in place.
Conclusion This is just one small test, and is by no means definitive, but it seems to indicate that there is a small correlation between the email message itself and email delivery issues.
Other issues that do affect delivery may be:

• Dirty email lists
• Not honoring unsubscribe requests
• Improperly configured email servers
• ISP or ESP sending limitations

The best way to pinpoint where email message delivery fails is to conduct tests to the available free email accounts as in this example.
Examine any bounces and errors closely to see exactly what triggered a failure.
Based on this simple test, troubleshooting time might be better spent with some of these other issues rather than with the email message itself.