since the email is going to them sometimes, but not others, and it is going out to other addresses, I would suspect it is either an issue with the SMTP Server not relaying the email to those domains for some reason, or that the email is being trapped in a spam folder.
you say that the email from the form is being used for the from address, i don't suggest doing it this way.
use a valid email from your domain as the from address, if you click the from button in the UE interface, in the advanced from settings, you can set the Return Address to use the dynamic address entered in the form.
doing it this way, you can people who may be getting the email trapped as spam add the address you are using for the from address to the address book or approved senders list to prevent it being treated as spam.
if you still have a problem, you will need to contact the host to look into the error logs for the smtp server to see why the email is not being relayed to those addresses.