[Dovecot] rare ssl issue

I'am having a rare situation with ssl connections. I can connect to 993/995 but the connection never starts.

From the client side i test with:

$ openssl s_client -connect example.com:993 CONNECTED(00000003)

and stays like that... waiting.

In the other side i see in dovecot logs:

Disconnected: Inactivity (no auth attempts): rip=x.x.x.x, lip=x.x.x.x, TLS handshaking

this happens from some networks only, so i think that mi isp (where the server is hosted) is doing some kind of shaping. But if i try to connect to port 465 for example, or 443 it works like a charm.

i am talking to my isp, but i thougth that someone may have a hint or any comment that could help.

regards,

eduardo.