The SMTP server is saved into user_projects/domains/bifoundation_domain/config/fmwconfig/biee-domain.xml , but there you will only find the server address and params, the username/password is probably stored in a secured wallet or something like that.
The SMTP username/password is not supposed to be mandatory (based on the doc and the interface at least), isn't it your browser which saved them and keep filling the form with the saved values?
You might want to remove the contents from above post as it is from the oracle support document.
Do as Ebin said and do not copy/paste content from MOS in the public forum. It's forbidden and a very strict rule as you have to pay to access MOS and so copy/paste giving it out for free is not really acceptable.
So edit your reply, just say if it fixed your issue (I'm sure it did) and then mark Ebin's as correct which will also close the thread (for now it's still This question is Not Answered.)