xxxxx.com domain cant send mail from mail client outlook ,but when try to check yyyyy.com domain can sending mail. Both domain located in same server. When i change xxxxx.com domain smtp port from 25 to 587 than it working and another domain yyyyy.com working default port 25. I cant understand why occurring this issue. xxxxx.com outlook error report Task 'firstname.lastname@example.org - Sending' reported error (0x80042109) : 'Outlook cannot connect to your outgoing (SMTP) e-mail server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).'