Looks like the problem is mail sending. Other instances are working correctly, but this one when try to send an email write this in event log
System.Net.Mail.SmtpException: Mailbox unavailable. The server response was: 5.1.0 DT1W1o00S0cxtzZ01T1krE authentication failed at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response) at System.Net.Mail.SmtpTransport.SendMail(MailAddress sender, MailAddressCollection recipients, String deliveryNotify, Boolean allowUnicode, SmtpFailedRecipientException& exception) at System.Net.Mail.SmtpClient.Send(MailMessage message) at DotNetNuke.Services.Mail.Mail.SendMail(String MailFrom, String MailTo, String Cc, String Bcc, String ReplyTo, MailPriority Priority, String Subject, MailFormat BodyFormat, Encoding BodyEncoding, String Body, List`1 Attachments, String SMTPServer, String SMTPAuthentication, String SMTPUsername, String SMTPPassword, Boolean SMTPEnableSSL)
Strange: the paramethers are ok, and working with other installations in the same server. in the host settings, SMTP settings, rewriting the login data and pressing test works fine the first time, gives error the second time.