Hugh

imho, it's the perception / context of the recipient that counts, not the sender - that's what people often forget about email, is that the recipient does not know the state of mind / context of the sender (if it's meant humourously, angrily, facetiously, etc, etc), so they can only read what they received, not what was "meant".

btw, how was it a "random person" that complained - surely it was the person who received and read the email (doesn't sound particularly "random" to me), and was offended by a swear word.