Missing e-mails

[#1]

I am using googlemail for sending and receiving e-mails but recently several people phoned me to say that whilst they got my e-mail ok I do not appear to have got their replies.
Have checked the spam box and they do not appear to have gone in there..... Is it common for e-mails to go missing or could something like a firewall or anti-virus be blocking them? If this was the case I assume they would be bounced back to the sender but it seems those sending me e-mails got no messages to say they had been bounced back to them.
Anyone able to throw any light on this matter ?
Thanks

[#2]

Have you received replies from these people in the past? It could be that you have your reply address configured incorrectly. Is there any commonality between the people you're not getting replies from i.e. are they all sending their email through the same server? How do you collect your mail, do you use a standalone client like Outlook? If so, check to see if your messages are there when you login to your mail at http://gmail.com.
If Google's servers are rejecting email from these senders then I would expect them to receive a failed delivery report. If Google is only temporarily rejecting the messages though, then the bounce can take several days to show up.

[#3]

Best way to test is to send an email to your self and see what you get then reply and see if it comes back ok. You can use gmail for all this but its better sending to another email address to be sure

[#4]

I Have an e-mail address with Tiscali as well so test  e-mails to myself from and to myself at googlemail appear to be arriving ok.
It just seems its the odd e-mail that is not arriving.... one person is using hotmail and these don't appear to be getting through ?
Just out of interest if someone sent me an e-mail with a virus(unknown to them) would it just be rejected at my end or would I get some warning message?
I do have a Firewall,virus checker and antispyware running on my machine.

[#5]

the PN antivurus will isolate the e-mail and send you an info e-mail.  A firewall wont stop a virus, your Anti Virus is a second line of defence