If you have multiple email addresses sending to a single mailbox it can be helpful to setup your mail client to allow you to send emails with multiple identities, which means that it would allow you at the time you send the message to choose which email address to send the email from. Here are the instructions for different mail programs:
Thunderbird – http://kb.mozillazine.org/Mozilla_Suite_:_FAQs_:_Mail_Aliases
Windows Live Mail – This only allows you to setup multiple inboxes, but that is fine. What you do is you set up inboxes for your other identities and tell it not to include them when downloading new mail. http://windows.microsoft.com/is-is/windows-live/mail-add-email-account-faq
If you are using IAM webmail then you will be using either HORDE, ROUNDCUBE or SQUIRREL MAIL.
If you have set up email fetching in gmail to download messages from the IAM mail server into your gmail account then you may need to setup multiple identities in gmail – https://support.google.com/mail/answer/22370?hl=en