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Setting up Sendy for a client and getting to cronjobs reminded me of an issue that could cause people tons of issues (not Sendy's fault!).
Some control panels use their own compile of PHP.
ServerPilot is one of them.
*/1 * * * * php /path/to/sendy/autoresponders.php > /dev/null 2>&1
In this install on a server using ServerPilot using php 5.6, I had to use:
*/1 * * * * php5.6-sp /path/to/sendy/autoresponders.php > /dev/null 2>&1
If your crons are failing, that's a good place to look.