We have no community rule on this, not even a "best practices" sort of thing... So as far as any dc game goes, it's perfectly legal.
However, GMs can write their own house rules on the matter. I do know of one GM that does so and prohibits the forwarding of mail.
Like you say here tho, (and like most tactics) it is best used when it's not expected. So building a reputation for such is terrible... but if you have a situation where you can do something creative w a forwarded, quoted, or doctored mail, and you think it will be received well or accomplish your goal, then by all means...