Setting the SMTP_OUT_PARAMETER in 10G at runtime
Looking at the docs, you cannot selectively choose from multiple SMTP_OUT_SERVER values. Here's how that parameter works:
http://docs.oracle.com/cd/B28359_01/server.111/b28320/initparams230.htm#REFRN10241

SMTP_OUT_SERVER specifies the SMTP host and port to which UTL_MAIL delivers out-bound E-mail. Multiple servers may be specified, separated by commas.
If the first server in the list is unavailable, then UTL_MAIL tries the second server, and so on.

It looks like you will need to manage this requirement externally.