Just want to expand on what @Yastiandrie has here. If you are on a POP3 account and you have setup the iPad to fetch automatically at certain intervals you may end up locking yourself out of using the email address on PC entirely.
This is because there is a window of ~15 minutes after you finish using the mail server before it will allow a connection from a different device. This means that if you have fetch set to 5, 10 or 15 minute intervals it will likely never allow you to login on PC.
When using a mobile device with a POP3 account it is best to set it to manual so you have to check your email rather than an automatic fetch.