Java EE (Java Enterprise Edition) General Discussion

We have deployed a Java application using JavaMail which enables people to send email to their contact lists. It has been in use for a few years, various updates being applied over that time. Most of our customers have no problem. But a couple of weeks ago, two of them suddenly could no longer send emails.

The connection to the SMTP server works but when the application sends email, it gets an error 421. A debugging dump is reproduced below: the target email address is changed to protect the uninvolved; messages starting with **** come from my program, other come from JavaMail.

Can anyone solve this problem? suggest where to look? Thanks.

DEBUG SMTP: useEhlo true, useAuth true
DEBUG SMTP: trying to connect to host "smtp10.bellnet.ca", port 25, isSSL false
220 toip35-bus.srvr.bell.ca ESMTP
DEBUG SMTP: connected to host "smtp10.bellnet.ca", port: 25
EHLO Communications
250-toip35-bus.srvr.bell.ca
250-8BITMIME
250 SIZE 20971520
DEBUG SMTP: Found extension "8BITMIME", arg ""
DEBUG SMTP: Found extension "SIZE", arg "20971520"
**** transport connect worked
NOOP
250 ok
**** about to send message to xxx@gmail.com
NOOP
250 ok
DEBUG: getProvider() returning javax.mail.Provider[TRANSPORT,smtp,com.sun.mail.smtp.SMTPTransport,Sun Microsystems, Inc]
DEBUG SMTP: useEhlo true, useAuth true
**** cEmailAuthenticator.getPasswordAuthentication() called
DEBUG SMTP: useEhlo true, useAuth true
DEBUG SMTP: trying to connect to host "smtp10.bellnet.ca", port 25, isSSL false
421 #4.4.5 Too many connections from your host.
DEBUG SMTP: could not connect to host "smtp10.bellnet.ca", port: 25, response: 4