SQL & PL/SQL

I have a PL/SQL block that repeatedly calls UTL_SMTP in a loop, basically e-mailing a group of addressees. The first send works fine (the message arriving successfully), but on the second and subsequent calls error 421 is raised by UTL_SMTP.HELO . My first and only guess was that messages were being sent too quickly for the mail server to process, so I inserted a 'sleep' of about five seconds between messages, but that didn't work.
I am using CLOSE_DATA and QUIT at the end of each iteration.

Many thanks,
Andrew Hunter