[jat850]

How does that get around a number of email clients flagging the warning that 'this email contains links to external images' or things of that nature? (Gmail does it, Thunderbird does it, I'm sure most modern email clients have that).

To me that would come across as, at best, annoying and at worst, suspicious.

[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.