If you're talking different A/V software, I use Avast! Antivirus on all of my Windows computers (the two that they are - the majority are Linux and one Mac).

If we're talking email, then here's a funny tip: Almost everything about an email can be spoofed. The way that servers identify if an email is from a spoofed host or not is by taking the IP address that the email originates from and grab the reverse data for it. If the reverse data matches the email's "@whatever.com" address, then it lets it pass. If it doesn't, then it'll probably drop it, or check the domain records of the correct reverse host for the "@whatever.com" domain to see if that mail server is authorized to deliver email for the domain.

The IP address *cannot* be spoofed. That's a server-to-server thing, not something that the sender can control. The IP address is the same address of the sending email server, which isn't something that can be changed with much ease and certainly can't be spoofed by a regular user.

The example in this case:
IP Address: 177.17.118.178
Reverse data for that IP address: 177.17.118.178.static.host.gvt.net.br (by the way, that host doesn't even exist, and "gvt.net.br" sends me to a Portuguese site, so something's iffy)
From address: yahoo.com
Should Yahoo's MX reject it or at least Junk-folder it: Yes.

This is what an actual message from Yahoo should look like:
MX [Mail Server] Non-authoritative answer:
yahoo.com mail exchanger = 1 mta6.am0.yahoodns.net. IP: 66.94.236.34
yahoo.com mail exchanger = 1 mta7.am0.yahoodns.net. IP: 66.94.237.64
yahoo.com mail exchanger = 1 mta5.am0.yahoodns.net. IP: 209.191.88.254

It's really hard for me to tell because Yahoo probably has a God-like amount of email servers (and that's probably just a few) but doing a reverse lookup on an IP should show something.yahoo.com, not what you got above.

I hope this helps you.

This is the fun knowledge that I get from hosting my own email server and doing computer system and network security. It's interesting, to say the least.