No problem, this is a good question. It's come up before. It has to do with the fact that many domains like GMail or Hotmail have what are called "SPF" records in their DNS. This is to prevent email spoofing and many spam filters check to see if the incoming message is being delivered from a server that falls in the range specified in the SPF record. The problem with OJS is that you (and others) are obviously not on the GMail network, and when you send email which uses a return email address from someone on a gmail.com or hotmail.com address, the spam filters on the recipient end will obviously pick up on this.
We're looking at ways to deal with this. For people who control their own domains, the solution is to include the IP address of the OJS sender in their SPF record. For folks with Gmail accounts, OJS needs to do something different. This will probably mean a tweak of the way mail headers are written in upcoming versions (perhaps a 'on behalf of' or setting a reply-to header instead). We're thinking about it.
In the meantime, have a look at docs/FAQ (the file bundled with OJS) and the allow_envelope_sender config.inc.php setting, which may help. Its the first question in the Advanced OJS Configuration part of the document.
Software Developer, Public Knowledge Project