I'm trying to help my father with an email problem he has.  He's using Outlook 2010 and has several email addresses set up, using a couple of different domain names that he owns, all going into the one mailbox at his website host.

In his Outlook he has an account and a folder set up for each address, and has rules set up to put incoming messages into the correct folder.

When he goes to send a message from any of his email addresses Outlook puts in the correct address in the From field, so this works ok.

When he goes to reply to a message he runs into problems.  On two of the addresses Outlook puts in the correct From address.  On the other two addresses Outlook puts in the wrong From address.

I'm not used to how Outlook works, I use Thunderbird myself, and so I'm struggling to find where in Outlook you set up which address to use in the From field when replying to a message.  I've gone into the settings for each account and for the life of me I can't find anywhere to set up a From address.  On the very first page of the account settings there's an Email Address field, and all accounts are set up with the correct email address here.  There's a Reply-To address field, but all accounts have this empty.  I can't find any other fields in the account where an email address can be put in.

Where do I set up the From address for an account?  In Thunderbird I'm used to using Identities, but I can't find anything like this in Outlook.

When replying he can click on the From button next to the From address and select the correct one, but we should be able to get Outlook to set it automatically.