Preferred way to embed pictures in e-mail

What is the preferred way to include pictures sent from e-mailing system. I mean automatic reports, newsletters and so on. I am interested in most compatible method unobtrusive for the final user.

I am thinking about two methods:

1. Hot-linking image from the server
2. Include image as an attachment and inline it using CID.

What are advantages and disadvantages of these me开发者_如何学Pythonthods and how do most common clients react to such scenarios? Is there any other or better way to include pictures.

Including the image(s) as an embedded resource usually leads to a better user experience, at least in my experience in the past. Mail clients are less likely to require the user to click on something to display the image(s), less likely to treat it as possible spam/phishing/etc.

The only downside I can think of, really, is the size of the resources. You'd want to keep the byte footprint of the image(s) as small as possible, of course. They'll consume company resources immediately while they're being sent out, rather than over time as the emails are read. Depending on how many emails need to go out, that could add up. It would also do more to fill a user's inbox. It's small, but it's aggregate. For a situation with a lot of internal recipients and small mailbox quotas, it could become an issue in time. But that's rare.

3. Hot-linking image from the server using SSL

It is important to use SSL since most web mail now use SSL by default, otherwise, user might receive a warning message to display unencrypted contents, e.g. using Gmail with IE8.