Sulfer smell

[daybyday]

Hi, I am new here at the forum though I have accessed Herb's Place for a long time.
Here goes.  Our basement drain backed up a few times last fall, we cleaned up, then had a pro. co. come in to snake out to the city water.  Just a bit of calcium buildup, no roots, I saw the camera run. House is 40 yrs. old.  He said there is a bow in the pipe under the washer, probably from years of settling.  
Since then, only when the first wash water drains into the tub there is a strong sulfer smell. Subsequent drainings have no odor.  It is not related to any other drain use in the house.
Any ideas why the smell?
Thanks,