The UDRP requires the Complainant to serve the Respondent when the complaint is filed.
[The Complaint must]
(xii) State that a copy of the complaint, together with the cover sheet as prescribed by the Provider's Supplemental Rules, has been sent or transmitted to the Respondent (domain-name holder), in accordance with Paragraph 2(b);
Now, paragraph 2(b) describes the various ways in which the Respondent is to be contacted, including by email. One would assume that if the Complaint is to "State" that it has been sent to the Respondent, then it would have to be sent to the Respondent when filed. But, if you want to be hyper-technical, no, it only has to say that it was.
Any person in the real world would assume that the Rule should be interpreted as requiring the Complainant to send a copy of the Complaint to the Respondent. You are, I suppose, free to differ.
However, the point is that when the Complainant complies (which they do pretty rarely these days, by the way), and the Respondent receives a copy of such Complaint, then both the date that the Complaint was sent, and the date that the Complaint was received are IRRELEVANT to the response deadline, which is the first thing you need to know when doing UDRP client intake. In fact, I never rely on any representation made by the client about dates, but contact the dispute resolution provider directly and get an authoritative response deadline from the dispute resolution provider. Because their "commencement date" is the date that counts. But even counting 20 days from the "commencement date" is not good enough.
Because even the dispute resolution providers count days and count the deadline for "submission" of a response differently. Many people do not realize that when you are running an international system with deadlines, that at any given moment, it is hardly ever the SAME day everywhere on the planet. When you say "Wednesday", do you mean Wednesday in Geneva, Minneapolis, Australia, or Hong Kong? If you mean "Wednesday" anywhere on the planet, then you are talking about a period of time which is potentially up to 48 hours long (from midnight in Hong Kong to midnight in Sydney).
On top of that, the NAF reads the Rules in a peculiar way which requires them to actually have received the entire response and exhibits by midnight in Minneapolis on the response date (unless the response date falls on a Saturday or Sunday - also not written in the rules). So, people who are east of Minneapolis have something of an advantage over people who are west of Minneapolis, in terms of getting it done and sent by fax. Of course, if you were thinking of sending it by courier, you will note that (unless they've changed it recently), the NAF only gives out a P.O. Box, and they do not publish a street address for courier delivery (they will provide it to you if you ask nicely).
In summary, the Rules themselves are deceptively simple, and there are quite a few "gotchas" in there if you haven't had the opportunity to apply those Rules in various situations.
And if you really want to get into the boring details of international deadline compliance from a fixed point on the globe, then you had better have a calendar with a good variety of holidays handy when you want to try to comply with the Rule 4(k) ten-day deadline, and you find that Victoria Day is approaching in the registrar's jurisdiction.