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My ISP does not allow SMTP port 25 connections outside its network but only port 587. I don't use a desktop mail client like Outlook.

Can a virus still send spam mail from my computer? How?

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They can send email on any port they want. SMTP traffic isn't limited to just port 25 thats just the default port. – Ramhound Feb 9 '13 at 20:13
You are right. But at least my computer will not send spam mail to big servers like yahoo and gmail etc and all those who use port 25 and 587 only. – Raouf Athar Feb 9 '13 at 20:47
Short answer to your YES, and there are many viruses out there so we all could not advise you. – mdpc Feb 9 '13 at 21:30
@RaoufAthar - These criminals don't use Yahoo and Gmail to send their spam their behavior is to easily blocked. – Ramhound Feb 10 '13 at 14:28

I knew some key-loggers that uses default browser for sending email, the approach is called injection (Injection-to-browser) which allowed by firewall to connect to outside. SMTP is one of the method for sending email, however, the virus might use HTTP protocol to send the email for example: sending parameters to remote host (remote PHP page), then, the host uses your IP for hiding the spammer's IP.

Another Scenario: if the virus got a copy of your SMTP credentials, then it can use it from another PC (Outside your pc).

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Nice answer but I did not understand the part where you say he will use my IP and hide the IP of php page computer. His php will execute somewhere else and not on my computer. – Raouf Athar Feb 9 '13 at 20:43
when you sent request from your browser (The browser sends headr also which contains hostname, IP, User-Agent, etc), then, the remote PHP page can used your IP at least put it in mail header. – Akam Feb 9 '13 at 20:46
Does he especially need my or your IP address? Can't he just randomly generate IP address and add it to this head. After all he is again sending it from his own computer and not mine. – Raouf Athar Feb 9 '13 at 20:52
@RaoufAthar: The important thing here is: you are involved legally not technically, this also another problem. – Akam Feb 10 '13 at 5:55

Yes. You do not need outlook to send an email, nor do you need a mail server - all you need is a connection to the Internet, a method of speaking tcp (telnet is one which comes with most PC's, but a virus would just hook into the TCP Stack using system calls), and knowledge of the public, and well understood mail protocol.

FWIW, any decent mail administrator can demonstrate how to do this - and most often has to to debug problems. (Un)fortunately sending an email is trivial, and requires nothing more then an Internet connection.

As far as your ISP's protections go, all a virus would need to do would be to discover the details of your ISP's mail server and "relay" mail through that server - just like outlook does. The advantage (to your ISP) is they can intercept the messages and check them for spam - and record the volumes, thus mitigating some of the risks.

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