@Ferrotter
 
@Background Pony #8598  
The cutie mark makes more sense as a snowflake, looking at the other three.
 
After all, the USPS creed is “Neither snow nor rain nor heat nor gloom of night stays these couriers from the swift completion of their appointed rounds.” One of the Derp-Ex ponies has a sun cutie mark (Heat), another has a raincloud cutie mark (Rain). I’m willing to bet that this one has a snowflake, to match.
 
The only problem is that Derpy doesn’t fit the pattern, and bubbles… doesn’t exactly match “Gloom of Night” that well- which doesn’t get a representative whatsoever. (Maybe the driver of the mail cart is a batpony with a moon cutie mark or something.) Then again, she existed well before these three ponies, so she almost certainly wouldn’t match it anyways.

With the white mane & tail, I’d say her cutie mark is a snowflake.

Yeah, I could’t tell for sure if it was a white flower with six petals, or a snowflake. That seems a reasonable facsimile of whatever it was.

She looks like a Pegasus Twilight with a white mane.

If anyone has name suggestions, please head to the background pony naming thread and post them there.