I use it, it works well for me.
There is nothing designed to make it delay though it may take a minute before the task starts.
If there are errors encountered in sending messages they are logged in the log.
Once the message is sent it is beyond the control of mojoPortal how long it takes to be received or whether it gets blocked by spam filters.
There can definitely be issues related to ISP and SMTP configuration that can affect how long it may take or the message can be rejected or caught by spam filters if the smtp server is spoofing an address for a different domain or there is no valid mx record pointing to the ip address or if the domain or ip address has been black listed by anyone for previous spam. ISPs may put limits on how many messages can be sent in a short time, etc etc.