avast treats each email in the same way, if it finds suspicious content or a virus it will warn you and you decide what to do with it. However, avast doesn't remember that email from this source or subject was suspicious or infected and block it, so the next time (tomorrow) you should get your email and avast will scan it as usual.
If avast does detect some suspicious (which I suspect is what is causing the warning), you will have to note what exactly is being said in the warning, so you can edit the heuristics settings.
But we need to know exactly what type of error or warning it was Suspicious or Infected message and the full wording to be able to pin down what actually detected it.
