Mail sending from wrong email address

Hello,

I'm wondering if anyone can help me with a mac Mail issues.

I have 3 email addresses set up on my Mac mail - 2 hotmail accounts and 1 account for my university email.

Of the 2 hotmail accounts, I use one for formal use e.g. job applications, and the other for everday mail e.g. newsletters, daily deals etc. I try to keep these as separate as possible so that I know where to look for things when I need them, and also because my 'formal' email address is my full name, and my everyday mail one is my nickname, nothing embarrassing but a bit silly and it just doesn't look very professional.

Anyway, the problem I'm having is that pretty much everytime I send a mail from my formal mail address, it shows in my formal addresses 'sent' folder, but when the person emails me back the reply is sent to my informal address and so that's what it must be sending on, even though it tells me otherwise.

For example, the other day I emailed someone about a job on my formal one, it said the message had sent from the formal address (even asked me for the password to my formal address to make the account 'online'), yet the woman replied to my informal address. This was the first time I'd ever contacted her so there's no way she could have had the informal address from before.

I'd really rather not send messages sent for professional purposes from an address with my nickname!

Does anybody know how I can stop this from happening? It's worth noting that this happens around 9 times out of 10, then every so often it does actually send from my formal address and i get a reply to the formal address. No idea why...

Any suggestions would be much appreciated!

Thanks

Hilary