Is storing SMTP details in Session object (IP, username, password) secure?

Hi.

I've written a tool for multiple companies to use.  Rather than it send emails from my SMTP server (support@mydomain.com) I want to let them use their own SMTP server (support@theirdomain.com) to send emails from.  I can store these settings in a SQL database.  My question is:

I can retrieve them once at login, and store them in a session object which they can use each time they send an email.  Is this secure?

Or

I could read the settings from the database each time they send an email (perhaps not as efficient?).

Thanks for any advice.