SMTP Connection Has Stopped Working

I have been having great trouble sending mails from my local Thunderbird/Windows client to my Debian/Exim-server via TLS/SMTP.

I am no expert on TLS and SMTP, but I managed to create a server certificate and set up a secure (TLS) connection more than a year ago, and it has been working fine until it suddenly stopped working a couple of weeks ago.

Sending to external addresses from web pages still works fine, the problem seems to be in the SMTP connection. If I try to "telnet xxx.xxx.xxx.xxx 25" I get no reply whatsoever (it seems to connect but it just gives me a blank screen with a blinking cursor) and I am wondering if there is a firewall issue or if I am simply not running the right daemon to listening for incoming SMTP connections...

POP continues to work fine with teapop/stunnel enabled.

I am running Exim 4.63-17 on Debian Etch. I configured Exim to be of the type "internet site; mail is sent and received directly using SMTP"

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Firewall shouldn't be a problem (?):

debian:~# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
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I really don't know what to do. I have tried running:
tail -f /var/log/daemon.log

Which responds when I access my mail using POP but not when I try to send using SMTP.