[stunnel-users] Incoming port ignored

Jose Alf. josealf at rocketmail.com
Thu Mar 31 05:22:29 CEST 2016

I checked the references. It looks like Avira works more or less as Ludolf thinks. Somehow, it intercepts connections to SMTP, POP3 and IMAP servers. The scan should be transparent to both mail client and server. If the traffic is encrypted between client and server, it can't scan it. 
Now, a connection can start in the standard (non-encrypted) ports and it can be upgraded to a secure one. If this happens, Avira blocks the connection. To avoid this, you must ensure your mail client communicates only in clear text. This is the crucial part. No SSL/TLS/STARTTLS allowed.

So, I think your workaround configuration should work. Set your accepts to (where port=25,110,143). This blocks connections from other machines to your stunnel service.
Configure your e-mail client to send mail via and fetch POP3 and IMAP Mail from and only with no encryption. Note: your mail client is NOT listening on those ports (stunnel is or will be listening). Your mail client connects to those ports.

Test as follows:
1. Disable Avira.2. If you have stunnel in service mode, make sure it is stopped.
2. Start stunnel in application mode. Make sure there are no errors. The log should tell you it is listening on ports 25,110,143. You can also use tcpview utility from sysinternals (now Microsoft) to verify this.
3. Try sending/receiving e-mail.4. If this works, enable Avira and test again.5. Report results.


On Wednesday, March 30, 2016 8:51 AM, Ivan De Masi <de_masi at blu-it.de> wrote:

I just tell Avira e-mail scanner on which ports it has to listen (POP3: 110
/ IMAP: 143 / SMTP: 25).
I can't configure any IP - but this is not necessary, because as I mentioned
before: When configuring the e-mail client with an unencrypted and direct
connection to my mailprovider, Avira is able to scan the e-mails. So it
already listens on localhost.

I found that workaround here:


And Outlook & Thunderbird are listening on,, ... it worked!!!  --- WRONG

I think from the moment I installed stunnel as a service problems started.
The servive-daemon also told me, that there is no config (?!).
So I switched back to the "GUI Start" and now it doesn't work any more :-/

Well, this seem logical to me, but when I switch off the mail-scanner it
doesn't interrupt the fetching or sending, only when I stopt stunnel e-mails
can't be fetched or send any more. So it seems to me somehow the mail-client
connects directly to stunnel? 

> Only the connection stunnel-provider will be encrypted.

Yes, that's right.


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