There are times in Outlook Express, when emails with the SMTP port 25 get rejected by RBLs like SpamCop and your own ISP’s public IP address is shown. That actually means that your own ISP’s public IP address has been blocked for the SMTP port 25.
For this, we have an alternate SMTP port on which no RBL check is implemented, i.e. 587. So, if you face such an issue then try to replace the default SMTP port 25/465 with 587 and then emails should be delivered just fine.
Other ports that are used with Outlook Express :-
SMTP Port = 25 or 587
SMTP Port over SSL = 465
POP3 = 110
POP3s = 995
IMAP = 143
IMAPS = 993
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