Mar 27, 2026 - For details about updated CVE-2025-53521 (BIG-IP APM vulnerability), refer to K000156741.

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FAJUMO's avatar
FAJUMO
Icon for Cirrus rankCirrus
May 08, 2024

Could not connect to SMTP host.

Getting the below error when i tried to query our SMTP server from the GUI SMTP Error: Could not connect to SMTP host. Same could connect when i tried it from the Cli
application delivery
Amine_Kadimi's avatar
Amine_Kadimi
Icon for MVP rankMVP
May 09, 2024

From CLI: telnet host 25

If you get no answer, then check the routing and make sure F5 has a route to the host if it is not directly connected. If routing is OK then check if there is any firewall in the path that is blocking the traffic

  • FAJUMO's avatar
    FAJUMO
    Icon for Cirrus rankCirrus
    May 09, 2024

    Hello Amine,

    Thanks for your swift response.

    I could telnet to port 25 to the said IP using CLI.

    but when i query from the GUI

    System  ››  Configuration : Device : SMTP  ››  SMTP_Host

    click test connection i get "SMTP Error: Could not connect to SMTP host."

    • FAJUMO's avatar
      FAJUMO
      Icon for Cirrus rankCirrus
      May 09, 2024

      SEE the output below
      14:10:14.630034 IP 10.158.11.74.25 > 10.161.6.6.60672: Flags [P.], seq 1:106, ack 
      14:10:24.625476 IP 10.158.11.74.25 > 10.161.6.6.33740: Flags [P.], seq 1:106, ack 2, win 8195, options [nop,nop,TS val 1601907323 ecr 1568290787], length 105: SMTP: 220 MBX01.etexample.com Microsoft ESMTP MAIL Service ready at Thu, 9 May 2024 14:10:23 +0100 in slot1/tmm1 lis=
      14:10:24.625559 IP 10.161.6.6.33740 > 10.158.11.74.25: Flags [R], seq 4003805261, win 0, length 0 out slot1/tmm1 lis=
      14:10:24.625728 IP 10.158.11.74.25 > 10.161.6.6.33740: Flags [F.], seq 106, ack 2, win 8195, options [nop,nop,TS val 1601907323 ecr 1568290787], length 0 in slot1/tmm1 lis=
      14:10:24.625736 IP 10.161.6.6.33740 > 10.158.11.74.25: Flags [R.], seq 2, ack 107, win 0, length 0 out slot1/tmm1 lis=

      • Amine_Kadimi's avatar
        Amine_Kadimi
        Icon for MVP rankMVP
        May 09, 2024

        The output confirms that F5 can indeed reach the host but the SMTP connection is reset at some point, did you confirm that actual SMTP delivery is not working (not the test button). If so, can you try to send an email using telnet connection and see if it gets through or if you receive any aditionnal message (for example, the server is not allowing you to send emails from/to some addresses).

        To send mail from telnet:

        1. Type telnet <server> 25
        2. Type HELO <domain>
        3. Type MAIL FROM: <email address>
        4. You may get a message saying “250 ok“
        5. Type RCPT TO: <email address>, <email address>, <email address>, etc.
        6. You may get another message saying “250 ok“
        7. To write the message, type DATA, followed by your message.
        8. To enter a Subject, type SUBJECT:, followed by your message.
        9. To end the message, put a period on a new line by itself and press Enter.
        10. Type QUIT to exit Telnet.

         

        credits: Using Telnet to Send Mail - Microsoft Consulting Services - CloudServus - United States