Some Internet Service Providers (ISPs') may block port tewnty-five for sending so try setting your machines up to send via port twenty-six to solve the sending issue. Your isp might also have service outages in your area dure to inclement weather or their servers could be over loaded (I.e send in the nine to five 8 hour time period) can some time yield dropped messages on isps such as AT&T and Bell South (DSL) and Comcast Cablevisoin and verizion (fiber and cable. The reason for port 25 being blocked inall consumer grade hogh speed Internet providers terms of service is to protect their customers form all the windows boxes running spam bots.
The main reason i mention this is that I have my own domain name that i use amoung other things for my email and my hosting provide hostmonster
uses port twenty-six as a work around for people like me who want to use their private mail server on hostmionster's system to send and receive mail however due to port twenty-five being blockedby default in the term of service for comcast and cablevision resdincial subcribers hostmonstewr has opened port twenty-six for less fortunate prople like my self whom can't afford the high monthly costs associated with the commercial class aof broad band internet access.
Digital Subscriber Line - A fornm of high speed inter net service provider of your land line providers network of coper RJ-11 phone cable.
FIOS- A residential grade of Fiberoptic inter netaccess provided by companies such as Verizon and AT&T , where said company brings fiberoptic cable to you residence.
Fiber optic A newer technology for transmiting data and voice communications over long distances in which the stream of analog or digital information is transmited at high speed over a network of cables made up of glass fiber.
Cable An older and in most cases cheaper method of high speed data transmission over long distances, in which Coaxial cable is used as the transmission medium. The vast majority of modern day cable Television and internet providers have fiberoptic based distribution networks.