Mercury - 421 Service not available, closing channel

[hinesystems]

I SOMETIMES get this error when trying to send authenticated mail via PHP to Mercury SMTP Server.

It appears quite rarely but when it does reports the service is unavailible for about 1 minute before it fixes itself.

Any ideas what I need to be checking?

Thanks



SMTP -> FROM SERVER:
250-****************; ESMTPs are:
250-TIME
250-SIZE 0
250-AUTH=LOGIN
250-AUTH CRAM-MD5 LOGIN
250 HELP
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "334 VXNlcm5hbWU6
"
SMTP -> get_lines(): $data is "334 VXNlcm5hbWU6
"
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "334 **********
"
SMTP -> get_lines(): $data is "334 **********
"
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "235 Authentication successful.
"
SMTP -> get_lines(): $data is "235 Authentication successful.
"
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "421 Service not available, closing channel.
"
SMTP -> get_lines(): $data is "421 Service not available, closing channel.
"
SMTP -> FROM SERVER:
421 Service not available, closing channel.
SMTP -> ERROR: MAIL not accepted from server: 421 Service not available, closing channel.