Contact form emails not coming through

Hello WordPress gurus,
I just found out that my contact form suddenly isn’t working properly (Contact Form 7 plugin). I’m not receiving emails sent through the form, though it says messages are sent successfully when submitting.
Following one suggestion, I installed the Mail-SMTP and set it to send all WordPress emails via SMTP, leaving all settings at default (“localhost” for STMP Host and “25” for the Port). The test email result says “bool(true)”, so I’m guessing it’s sending OK, but it still doesn’t show up in my email or spam folder.
I’m pretty much stumped at this point. Could it be a problem with the hosting site (Gate.com)? Or should I be looking elsewhere? I haven’t changed anything recently. I’m still running WP 3.1.3. I did add a Pinterest button plugin fairly recently, but tried deactivating that to no avail, so that doesn’t seem to be the problem.
Thanks for any and all help.