Since they were using multiple devices, the obvious solution was to use IMAP if possible. Using IMAP means all devices see the same emails. With POP3 each device downloads emails and it can get a bit messy deleting emails as the same email needs to be deleted on each device. I prefer to set up email using SSL with the password being encrypted. This provides better security when using third party access points such as when travelling.
A quick check on the Optus site and the email settings didn't mention SSL, so I decided to check the internet. As usual there's a lot of misinformation with sites providing values for SSL for use with Optus. I tested the SSL port settings found on the internet crowd support sites and they did not work with Optus.
The Optus help does state that port 110 is used for IMAP/POP3 for incoming emails and port 25 for SMTP for sending emails. The server's address is mail.optusnet.com.au. This works, but there is no security, as in encryption of passwords.
If you're connecting directly to the Optus service using their fixed or mobile broadband this shouldn't be a problem. However, if you're travelling and using a third party access point to connect to your email, this is a real concern. Whilst travelling and using third party access points, it is perhaps best to use webmail, which will give you better security. Alternately think about using a service such as Gmail which provides better security for email clients.