While sending mail from oracle, if there is no error in SMTP then there will not be any problem.
If the SMTP gives error like invalid email id, relay not permitted for oracle server IP then oracle does not giving error message. Always it is trying the same process and oracle.exe memory is getting increased in oracle server.
Due to this, all users will get out of memory error. If this error comes, immediatly we need to check and stop all email services from oracle. This error/bug is available from oracle 8i onwards to 10g R2.