You should not be relying on a free anonymous mail provider for business critical email. It looks unprofessional and frequently results in anomalies such as what you are experiencing now.

That aside, we recently experienced a failure of outgoing mail send from bots in our corporate site (hosted by SiteGround). It turned out to be SiteGround that had instituted a block on all outgoing mail not whitelisted in their anti-spam filter rules. Took a day to figure that out, though.