Problem Sending Mails With Smtp Server

[metaldallen]

PHP version: 5.4.16
Prestashop version: 1.5.6.2

 

Neither the contact form or the Test-mail function works. the test mail functions return the following error message:

Error: Please check your configuration

There was a problem reading line 1 of an SMTP response. The response so far was:

It appears the connection has died without saying goodbye to us! Too many emails in one go perhaps? (fsockopen: #0) .

There are no PHP error messages although PS_MODE_DEV is set to true.

It seems not to be a firewall problem, since other sites hosted at the same web server are able to send mails using the same SMTP server and the same login credentials.

From the mail server log file:

connect from xxx
lost connection after connect from xxx
disconnect from xxx

We can't see any error messages.