And receiving them depends on several things (including, but not limited to):

• A cooperating mail client; that is, the receiving mail client is set¹ (or is permitted²) to return those when requested.
• There is nothing that filters them³.

So if you want a guarantee⁴ that they opened it, call them up and ask.

Notes:

1. Most mail client programs I am familiar with have settings which allow you to decide what to do with those requests.
2. I have set my mail client to ask, and generally prohibit it from responding to those requests.
3. I have seen bounce messages from spam or other filtering systems where the requested responses were blocked by the server the requester used, for whatever reason.
4. The use of "web bugs" and the like (see also) also may not work, as some mail clients can be configured to not display images^{5, 6}, and the presence of a "web bug" may give your email a better chance of being caught by spam filtering.
5. I have that disabled as well.
6. Then there are also mail clients like pine and mutt, which run in console sessions.