Dutch users of English often include hereby or herewith in the opening sentence of their business letters or emails. In English, however, these words are very formal and are normally only used in legal documents. So unless you’re a lawyer, it’s better to find some other, more friendly formulation!
For example, the following are all good opening sentences that avoid using hereby or herewith:
I write to confirm that I have received…
As requested, I am sending you…
I am pleased to enclose a copy of our latest…
As we agreed on the phone this afternoon, I am sending you…
©2013 Baxter Publishing, Hilversum, The Netherlands