Mail mixing up sending between my 2 different email addresses

I am using two different mail addresses, one a gmail address which I us for private mails, and one my corporate address.

The latter is also with Gmail, through Google apps.

Both have the same outgoing smtp, namely smtp.gmail.com

Now when I want to send an email I check from which address it is sending my mail.

Still I get replies from mails sent with my corporate mails back to my private mail address.

Asking a co worker, he confirmed that he receives my mails from my private address, even though I carefully checked that in the "from" it shows my coporate mail address.

Does anybody have had the similar problem, and found a solution?