What does this mean?
The problem is when I tried to send email being connected to my LAN.
Send email with what program? To where?
There are two ways email servers "send and receive" email.
1) Email client (Outlook and Thunderbird) to and from the email server (Sendmail or Postfix).
2) Email server (Sendmail or Postfix) to and from another email server (Sendmail or Postfix).
Email clients communicate with the email server for POP3/IMAP (receive email from server) and SMTP (send email to server).
POP3 uses port 110 or 995
IMAP uses port 143 or 993
SMTP uses port 25 or 465
Email servers communicate with other email servers using port 25.
Which of those are failing?