Where an enactment authorises or requires a document to be served by post, by using the word "serve", "give", "deliver", "send" or any other word or expression, the service of the document may be effected by properly addressing, prepaying (where required) and posting a letter containing the document, and in that case the service of the document is deemed, unless the contrary is proved, to have been effected at the time at which the letter would be delivered in the ordinary course of post.