Email relay from contact form on DO droplet not reaching external SMTP relay
I have a Magento-based website hosted on a Digital Ocean droplet with multiple contact forms. The droplet runs Ubuntu 14.04.2 LTS and has postfix configured with DKIM and Mitel, which originally relayed to a Gmail account directly. About a year ago, these stopped working, and after consulting with an outside SEO service that was helping to manage the site, the droplet/Postfix were reconfigured to connect to a TurboSMTP account, which in turn relayed to the Gmail account. This stopped working last Friday. Based on the TurboSMTP logs, it seems the emails aren't even getting from the website to TurboSMTP. The information on the contact forms is visible on the Magento admin page, and can be relayed manually from there. I've looked at the mail logs, and can't even find those contact form entries in the logs (then again, the info could be in the logs and I'm just not seeing it). I check with the SEO/developer mentioned above, and they haven't changed anything on the back end (at least nothing that's gone live). The latest change was to add a new page to the site for a new product, but that's been live for over a month. I can access the Gmail account fine, and there's no indications of any rejections there. I rebooted the droplet yesterday, but test entries with the contact forms have not yielded success (though I will try again after sending this).
I'm wondering if anyone can offer any other suggestions for troubleshooting this email issue (I would have asked sooner but the DO site was down earlier this morning). I can submit log samples or complete logs if further information is needed.
Thanks in advance,