Avoiding the chance to say "yeah, it's our competitor's problem!" here...
It's possible that is the issue, but it's by no means certain, and I would go so far as to say it's highly unlikely.
A couple possibilities below. Take them with the knowledge that I haven't seen the email in question and so can't say whether these are the case; they're based on my experiences.
1. The most likely culprit is that the destination URL is displaying in the message, but the subscriber passes through a tracking URL first.
This is phishing in a nutshell: the link looks like it's taking you one place immediately, but it goes to another.
Solution: use linked text like "Click Here" or something else instead of simply showing the destination URL.
2. There's other content in the email that makes the message seem similar to other emails that Gmail's algorithms or team has associated with phishing.
This is a pretty vague scenario, so here's one specific thing to look for: is your client using a public link shortener for links in his/her newsletter? If so, don't do that. These are open to abuse by spammers & phishers, and your emails can potentically get blocked/filtered if you are using the same tools as those guys. More on this here:
Are Blacklisted Link Shorteners Getting Your Emails Blocked?
Again, hard to say without seeing the email in question, but these are common mistakes that people make when sending and tracking email campaigns.
Hope this helps!
Cheers,
Justin Premick
Director of Education Marketing
AWeber Communications