Why are some emails not being sent to certain members of a group?

I send a group email every week. There are two members of the group, however, who are not receiving the emails, even though, outlook tells me that the messages were successfully sent. It pisses me off, because I can't rely on outlook to tell me the truth of weather or not they were ACTUALLY successfully sent. People have to be be able to trust that their emails were sent when outlook says they were. This has not just happened with group messages but also with individual messages I've sent. One time, I sent an email 5 times to one person before he actually received ONLY ONE of the multiple emails that were sent. This is unacceptable. I really want to switch over to gmail. It's just because I alot of my accounts are linked to this email and I'd have to go through and change all of them. UGH, I am just so frustrated with outlook right now, because I don't know who is actually receiving my emails and who is not, (despite outlooks ever optimistic and untruthful assurance that "you're message has been successfully sent") If my message didn't send, that's fine, but outlook should, at least, be accurate in their assessment of weather it did or it didn't, so that I can know weather the email needs to be sent again. It's freaking ridiculous. Outlook sucks. Has anyone else experienced this problem?