All our emails were soft bouncing and we couldn't figure out why.
After a bit of investigation we found it was because of a very minor typo in our standard footer.
One of our dev's had put a color value in as ##0000ff instead of #0000ff.
The double hash caused Marketo to think it was parsing a token value.
The failure to parse that value caused Marketo to reject the emails; throwing out a Soft Bounce.
Thought it would be useful to have this documented in the community.