Why can’t we send mail through SMTP ?

About “Port 25” blocking
Many ISP's (Internet Service Providers) are blocking what is called "Port 25" which is the port used to send e-mail. They have chosen to do this to cut down on the amount of "SPAM" mail that is sent. All e-mail sent via the Internet is routed through the port 25, the channel used for communication between a mail client and a mail server. Even though port 25 blocking will probably become an industry standard, however, the filter can create problems for e-mail servers and block legitimate e-mail as well as spam. You can check with your dial-up provider to see if they are doing this. The way around this is to set your SMTP settings to those of your ISP's. They can provide the settings for you to send email via their SMTP server. For example: "mail.aol.com". After doing that, you will be sending mail though your ISP's servers rather than the SMTP server on your computer. Unfortunately, you will not be able to use any SMTP mail server software, including your private server or hosting account.
Here are a few of the major ISP's that block Port 25:
• NetZero
• Mindspring
• MSN
• Earthlink
• Flashnet
• MediaOne
• AT&T
• Verizon