Piattaforma email-marketing

[Faxio]

Ciao ragazzi,
ho realizzato una piattaforma di invio tutta mia.
Attualmente, per l'invio utilizzo uno script php (schedulato con corntab) che a sua volta utilizza la classe phpmailer.
5 server SMTP si occupano dell'invio delle email.
Questi server li gestisco attraverso Plesk e sono configurati tutti allo stesso modo.
Solo su uno di questi, non riesco ad effettuare la connessione SMTP, phpmailer mi restituisce il seguente log:

SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "220 srv01.mydomain.com ESMTP "
SMTP -> get_lines(): $data is "220 srv01.mydomain.com ESMTP "
SMTP -> FROM SERVER: 220 srv01.mydomain.com ESMTP
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "250-srv01.mydomain.com "
SMTP -> get_lines(): $data is "250-srv01.mydomain.com "
SMTP -> get_lines(): $data was "250-srv01.mydomain.com "
SMTP -> get_lines(): $str is "250-AUTH=LOGIN CRAM-MD5 PLAIN "
SMTP -> get_lines(): $data is "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN "
SMTP -> get_lines(): $data was "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN "
SMTP -> get_lines(): $str is "250-AUTH LOGIN CRAM-MD5 PLAIN "
SMTP -> get_lines(): $data is "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN "
SMTP -> get_lines(): $data was "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN "
SMTP -> get_lines(): $str is "250-STARTTLS "
SMTP -> get_lines(): $data is "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN 250-STARTTLS "
SMTP -> get_lines(): $data was "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN 250-STARTTLS "
SMTP -> get_lines(): $str is "250-PIPELINING "
SMTP -> get_lines(): $data is "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN 250-STARTTLS 250-PIPELINING "
SMTP -> get_lines(): $data was "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN 250-STARTTLS 250-PIPELINING "
SMTP -> get_lines(): $str is "250 8BITMIME "
SMTP -> get_lines(): $data is "250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN 250-STARTTLS 250-PIPELINING 250 8BITMIME "
SMTP -> FROM SERVER: 250-srv01.mydomain.com 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-AUTH LOGIN CRAM-MD5 PLAIN 250-STARTTLS 250-PIPELINING 250 8BITMIME
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 UGFzc3dvcmQ6 "
SMTP -> get_lines(): $data is "334 UGFzc3dvcmQ6 "
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "535 auth failure "
SMTP -> get_lines(): $data is "535 auth failure "
SMTP -> ERROR: Password not accepted from server: 535 auth failure
SMTP -> get_lines(): $data was ""
SMTP -> get_lines(): $str is "250 flushed "
SMTP -> get_lines(): $data is "250 flushed "
SMTP -> FROM SERVER: 250 flushed

Per connettermi, utilizzo un utente di posta di un dominio configurato sul server.
Avete idea di dove possa essere l'errore?
Il mio provider già mi ha risposto che non impongono particolari restrizioni per le connessioni dall'esterno, inoltre preciso che questo tipo di configurazione la utilizzo senza problemi su altri server.

Grazie

[Provatoo]

Sei sicuro che il tuo script funzioni?

[Faxio]

Lo script funziona benissimo per tutti gli altri server, solo per uno mi da l'errore di autenticazione che ho descritto