Question

The error seems to be very non-descriptive:

failed with ArgumentError: A sender (Return-Path, Sender or From) required to send a message 

I think this is not the real problem since I’ve tried specifying “from” even though I set a default. Also, the code is identical for localhost and Heroku…

def invite_dealer(auction, name, email, dealer)
  return false if dealer.bids.where("auction_id = ?", auction.id).present?
  @name = name  #used on template
  @email = email  
  @auction = auction  #this too
  mail(:to => @email, :subject => "New Auction - #{auction.car.name}", :from => "[e-mail]")
end

Answer 1

The error failed with ArgumentError: A sender (Return-Path, Sender or From) required to send a message can occur if mail method was not invoked from invite_dealer method.

So putting conditional return into invite_dealer method before call to mail method is bad idea.