In the past years, more and more internet service providers started with the unsocial behavior, to block the standard SMTP port (25) - mostly without notifying the customers. I've always been very skeptical about this blocking and I was glad it seemed that there are working real technicians at my current ADSL provider (Swisscom) who know about the importance of using standard (and RFC!) ports. Until yesterday.
While trying to send an e-mail from my Android phone (with K9 mail app) yesterday using my Wireless LAN at home, which uses the ADSL connection from Swisscom, I received the following error message from Swisscom, I have to change ports to 587.
573 Swisscom Antispam: Authentifizierte Verbindung nicht moeglich. Bitte benutzen Sie den Port 587 anstelle von Port 25. Weitere Informationen: www.swisscom.ch/p25.
Connexion authentifiee pas possible. Veuillez utiliser le port 587 a la place du port 25. Ulterieurs informations: www.swisscom.ch/p25.
Collegamento autenticato non e possibile. Si prega di utilizzare la porta 587 invece di porta 25. Altra informazione: www.swisscom.ch/p25.
Authenticated connection is not possible. Please use port 587 instead of port 25. More information: www.swisscom.ch/p25.
Luckily I'm a technician so I know what this means, but now imagine this for the rest of the customers of Swisscom. 95% wouldn't know what this means and would start to blame anything else.
Oh and by the way - this only affects the ADSL/fixed line connections yet. So far no problems to send e-mails on port 25 via 3G/EDGE/HSDPA.