I install Mail Server plugin in my panel. I can receive emails, but when I try to send mail, I get this error message:
Theme:Undelivered Mail Returned to Sender
when I test with https://www.mail-tester.com/ get this errors

Hello, please check whether the server's port 25 communicates normally, and the normal communication of port 25 is required to send mail.

    kohestanimahdi No, no, it just shows that it doesn't work when the system shows that it runs. You must be able to communicate with it. For example, telnet ipaddress 25 can communicate.

      aaPanel_thisyear Hello,
      No, when I try to send mail, mail returns to my inbox. I check spam and trash in the receiver's email address.

      I can receive email from my google email, but can not send any mail

        10 days later

        I am having same issue. I can receive email, but cannot send them I don't get any error as reply, but it just shows a pop up this error:

        "Some cloud vendors (such as Google, Amazon) close port 25 by default, and you need to contact the vendor to open port 25 before you can use the post office service normally"

        However, I check Port 25 is open and communicating and I am also running ubuntu

          kohestanimahdi
          Please follow the tips of mailtest to improve your information. If your email score is too low, it may be rejected by other inboxes or put in the trash.

          2 months later
          a year later

          aaP_toolzneeded456
          Hello, please describe your problem in detail. If the server cannot connect to port 25 of other servers, check whether the server provider has disabled port 25

          12 days later

          Some cloud vendors (such as Google, Amazon) close port 25 by default, and you need to contact the vendor to open port 25 before you can use the post office service normally please help

            aaP_ssquadram
            hi,
            We can't solve this problem. It is recommended to contact the server provider
            Or use smtp trunk

            10 days later

            I am also face same problem, I received mails but unable to send mail is anybody here to help me

              Dear @"aaPanel_Kern" I am using aapanel server My server received mail from other mail but when I try to send mail outside, this shows your message sent successfully but the mail could not reach the given address.

              Nov 25 05:33:11 webcarehouse postfix/smtpd[27757]: disconnect from unknown[87.246.7.78] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:19 webcarehouse postfix/smtpd[27751]: warning: hostname net6-ip78.linkbg.com does not resolve to address 87.246.7.78: Name or service not known
              Nov 25 05:33:19 webcarehouse postfix/smtpd[27751]: connect from unknown[87.246.7.78]
              Nov 25 05:33:20 webcarehouse postfix/smtpd[28724]: warning: unknown[87.246.7.227]: SASL LOGIN authentication failed: Connection lost to authentication server
              Nov 25 05:33:20 webcarehouse postfix/smtpd[28724]: disconnect from unknown[87.246.7.227] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:22 webcarehouse postfix/smtpd[27600]: warning: hostname net6-ip227.linkbg.com does not resolve to address 87.246.7.227: Name or service not known
              Nov 25 05:33:22 webcarehouse postfix/smtpd[27600]: connect from unknown[87.246.7.227]
              Nov 25 05:33:29 webcarehouse postfix/smtpd[27600]: warning: unknown[87.246.7.227]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:29 webcarehouse postfix/smtpd[27751]: warning: unknown[87.246.7.78]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:29 webcarehouse postfix/smtpd[27751]: disconnect from unknown[87.246.7.78] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:30 webcarehouse postfix/smtpd[27600]: disconnect from unknown[87.246.7.227] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:37 webcarehouse postfix/smtpd[27757]: warning: hostname net6-ip227.linkbg.com does not resolve to address 87.246.7.227: Name or service not known
              Nov 25 05:33:37 webcarehouse postfix/smtpd[27757]: connect from unknown[87.246.7.227]
              Nov 25 05:33:38 webcarehouse postfix/smtpd[28724]: warning: hostname net6-ip78.linkbg.com does not resolve to address 87.246.7.78: Name or service not known
              Nov 25 05:33:38 webcarehouse postfix/smtpd[28724]: connect from unknown[87.246.7.78]
              Nov 25 05:33:39 webcarehouse postfix/smtpd[27751]: connect from unknown[141.98.10.153]
              Nov 25 05:33:39 webcarehouse postfix/smtpd[27751]: disconnect from unknown[141.98.10.153] quit=1 commands=1
              Nov 25 05:33:43 webcarehouse dovecot: auth: Warning: auth client 0 disconnected with 1 pending requests: Connection reset by peer
              Nov 25 05:33:48 webcarehouse postfix/smtpd[27757]: warning: unknown[87.246.7.227]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:49 webcarehouse postfix/smtpd[27600]: connect from unknown[141.98.10.153]
              Nov 25 05:33:49 webcarehouse postfix/smtpd[28724]: warning: unknown[87.246.7.78]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:49 webcarehouse postfix/smtpd[27757]: disconnect from unknown[87.246.7.227] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:50 webcarehouse postfix/smtpd[28724]: disconnect from unknown[87.246.7.78] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:52 webcarehouse postfix/smtpd[27751]: warning: hostname net6-ip227.linkbg.com does not resolve to address 87.246.7.227: Name or service not known
              Nov 25 05:33:52 webcarehouse postfix/smtpd[27751]: connect from unknown[87.246.7.227]
              Nov 25 05:33:55 webcarehouse postfix/smtpd[27600]: warning: unknown[141.98.10.153]: SASL LOGIN authentication failed: UGFzc3dvcmQ6

              Dear @aaPanel_Kern I am using aapanel server My server received mail from other mail but when I try to send mail outside, this shows your message sent successfully but the mail could not reach the given address.

              Nov 25 05:33:11 webcarehouse postfix/smtpd[27757]: disconnect from unknown[87.246.7.78] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:19 webcarehouse postfix/smtpd[27751]: warning: hostname net6-ip78.linkbg.com does not resolve to address 87.246.7.78: Name or service not known
              Nov 25 05:33:19 webcarehouse postfix/smtpd[27751]: connect from unknown[87.246.7.78]
              Nov 25 05:33:20 webcarehouse postfix/smtpd[28724]: warning: unknown[87.246.7.227]: SASL LOGIN authentication failed: Connection lost to authentication server
              Nov 25 05:33:20 webcarehouse postfix/smtpd[28724]: disconnect from unknown[87.246.7.227] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:22 webcarehouse postfix/smtpd[27600]: warning: hostname net6-ip227.linkbg.com does not resolve to address 87.246.7.227: Name or service not known
              Nov 25 05:33:22 webcarehouse postfix/smtpd[27600]: connect from unknown[87.246.7.227]
              Nov 25 05:33:29 webcarehouse postfix/smtpd[27600]: warning: unknown[87.246.7.227]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:29 webcarehouse postfix/smtpd[27751]: warning: unknown[87.246.7.78]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:29 webcarehouse postfix/smtpd[27751]: disconnect from unknown[87.246.7.78] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:30 webcarehouse postfix/smtpd[27600]: disconnect from unknown[87.246.7.227] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:37 webcarehouse postfix/smtpd[27757]: warning: hostname net6-ip227.linkbg.com does not resolve to address 87.246.7.227: Name or service not known
              Nov 25 05:33:37 webcarehouse postfix/smtpd[27757]: connect from unknown[87.246.7.227]
              Nov 25 05:33:38 webcarehouse postfix/smtpd[28724]: warning: hostname net6-ip78.linkbg.com does not resolve to address 87.246.7.78: Name or service not known
              Nov 25 05:33:38 webcarehouse postfix/smtpd[28724]: connect from unknown[87.246.7.78]
              Nov 25 05:33:39 webcarehouse postfix/smtpd[27751]: connect from unknown[141.98.10.153]
              Nov 25 05:33:39 webcarehouse postfix/smtpd[27751]: disconnect from unknown[141.98.10.153] quit=1 commands=1
              Nov 25 05:33:43 webcarehouse dovecot: auth: Warning: auth client 0 disconnected with 1 pending requests: Connection reset by peer
              Nov 25 05:33:48 webcarehouse postfix/smtpd[27757]: warning: unknown[87.246.7.227]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:49 webcarehouse postfix/smtpd[27600]: connect from unknown[141.98.10.153]
              Nov 25 05:33:49 webcarehouse postfix/smtpd[28724]: warning: unknown[87.246.7.78]: SASL LOGIN authentication failed: UGFzc3dvcmQ6
              Nov 25 05:33:49 webcarehouse postfix/smtpd[27757]: disconnect from unknown[87.246.7.227] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:50 webcarehouse postfix/smtpd[28724]: disconnect from unknown[87.246.7.78] ehlo=1 auth=0/1 rset=1 quit=1 commands=3/4
              Nov 25 05:33:52 webcarehouse postfix/smtpd[27751]: warning: hostname net6-ip227.linkbg.com does not resolve to address 87.246.7.227: Name or service not known
              Nov 25 05:33:52 webcarehouse postfix/smtpd[27751]: connect from unknown[87.246.7.227]
              Nov 25 05:33:55 webcarehouse postfix/smtpd[27600]: warning: unknown[141.98.10.153]: SASL LOGIN authentication failed: UGFzc3dvcmQ6

                aaP_iamshoaib216
                Prompt: SASL LOGIN authentication failed
                Check whether the email account password is correct. You are advised to reset the password once