The flaw in this, is that it puts the onus on you to decide what 'may' be offensive.
As the law has demonstrated that it is unable to make that distinction on many occasions, why should young TMR be able to do so?
If your wife were coloured, and you had no problems calling her and all her family 'nigger' to their faces, why would you think a 'nigger' joke in an e-mail was offensive?
If they can't be precise about what they mean, then your subjective judgement is worthless.
You could go the other way and bombard them with every e-mail you ever get, on the grounds that it may be offensive to someone, although you don't know who.