[lftl]

Instead of an invisible tracking image make it part of your signature, like a company logo or something similar.

[wnight]

You'd need to give it a distinct filename or something, but it's otherwise fairly reasonable. But I still wouldn't see it because I've got all external images turned off. And then you'd either need a redundant text sig or risk clients not having your phone number, etc.

I email clients a passworded URL they can download documents from instead of attaching the documents. Partly so I can send someone files of any size and without attachment screening, and partly for delivery tracking.