In the pinned overview from David above he gives the following URL as an example of the "light text branding" you get with Tiers 3–5: https://www.dropbox.com/s/zi2hcz0sxgomssm/SendFox%20Light%20Branding%20Example.png?dl=0

In reality, the text branding no longer looks like that. Instead, at least in Tier 3 account I just purchased, there's only a minimal difference in the branding between the various tiers.

With a free account the branding appears as a 66x66 px image with the words "POWERED BY SENDFOX" in light gray forming a circle, with the fox and envelope logo in the middle of the circle; this image appears in the bottom right corner of the email. The Tiers 3–5 branding is the same "POWERED BY SENDFOX" circle, but without the fox and envelope in the middle of it. While SendFox can argue that they've technically stuck with the terms of the sale — the Tiers 3–5 branding does contain only text in the image — it is nonetheless significantly more obtrusive than the example they provide in their description.

This discrepancy between the branding they show in their sample, and the branding you actually get with a subscription, does make me concerned that they'll make more unilateral changes to my emails in order to push their product.