Overall membermeister has an exceptionally good delivery rate and we deliver close to 100% of all messages successfully. However a successful delivery only means that the email from membermeister has been handed to the recipient's provider, for example GMail. It does not mean that the recipient sees the email because filters, rules or other issues can occur after the email is out of membermeister's control. Other times the delivery can be delayed.
As a result you may see messages that show as delivered but are never opened. Or you may have a recipient tell you they 'did not receive the email' when in actual fact they have received it but not seen it.
Generally, the speed of delivery depends largely on the receiving account, and how quickly they will receive and process emails. Most of the major email providers such as Hotmail, AOL or Yahoo limit how much email they'll receive in a certain period of time and will restrict delivery for a period of time.
Additionally, even though membermeister will hand the messages off to other providers for delivery almost immediately, it's still up to that provider to actually process that email and place it in the recipient's inbox.
Each receiving server is different though, so for example, it may take a different amount of time for Hotmail to process the mail than Gmail. In many cases, things like the time of day and overall email traffic to that recipient server can affect how quickly they're able to receive and process email.
This means that sometimes emails may be queued rather than delivered immediately.
Therefore it is possible that even a successfully delivered message which did not bounce has not yet been actually reach the recipient's inbox.