unable to login to mailserver via outlook?

Some clients have been complaining unable to login to mailserver via outlook, most of them are showing this error:

An unknown error has occurred. Subject 'XXX', Account: 'mail.XXX.net', Server: 'mail.xxx.net', Protocol: SMTP, Port: 25, Secure(SSL): No, Error Number: 0x800CCC0B

While I have no problem logging in, it happens once in a while, any idea?

 

 

 

 

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