I have the same problem with and without extension plugin. My error log: ------------ ERROR #43 ------------TYPE: MAILERRORS: {\"wp_mail_failed\":[\"You must provide at least one recipient email address.\"]}ERRORS_DATA: {\"wp_mail_failed\":{\"to\":[\"\"],\"subject\":\"Id\\u0151pont foglal\\u00e1s Dr Kov\\u00e1cs Norbert neurol\\u00f3giai mag\\u00e1nrendel\\u00e9s\\u00e9re 42\",\"message\":\"<p>canceled<\\/p>\\n<p>Kedves <span id=\\\"custom-tags\\\">\\u00dajabb Pr\\u00f3ba v\\u00e1s\\u00e1rl\\u00f3!<\\/span><\\/p>\\n<p>K\\u00f6sz\\u00f6nettel vett\\u00fck bejelntkez\\u00e9s\\u00e9t Dr. Kov\\u00e1cs Norbert neurol\\u00f3gia mag\\u00e1nrendel\\u00e9s\\u00e9re.<\\/p>\\n<p><em>A lefoglalt vizsg\\u00e1lat f\\u0151bb adatai:<\\/em><\\/p>\\n<p>Vizsg\\u00e1lat helye: <strong>P\\u00e9cs Mag\\u00e1nrendel\\u00e9s, 7621 P\\u00e9cs M\\u00e1ty\\u00e1s kir\\u00e1ly utca 6, P\\u00e9cs Mag\\u00e1nrendel\\u00e9s<\\/strong><\\/p>\\n<p>Vizsg\\u00e1lat id\\u0151pontja: <strong>2017-12-27, 16:00 <br \\/><\\/strong><\\/p>\\n<p>Id\\u0151pont foglal\\u00e1s azonos\\u00edt\\u00f3ja \\u00e9s st\\u00e1tusza: <strong>42, lemondott<\\/strong><\\/p>\\n<p><em>Az \\u00d6n \\u00e1ltal megadott adatok:<\\/em><\\/p>\\n<p><span id=\\\"custom-tags\\\">-N\\u00e9v: <strong>\\u00dajabb Pr\\u00f3ba v\\u00e1s\\u00e1rl\\u00f3,<\\/strong><\\/span><\\/p>\\n<p><span id=\\\"custom-tags\\\">-Email c\\u00edm: <strong>neurologykft@gmail.com <\\/strong> <\\/span><\\/p>\\n<p><span id=\\\"custom-tags\\\">-Telefonsz\\u00e1m: <strong>+36702221178 <\\/strong><\\/span><\\/p>\\n<p><span id=\\\"custom-tags\\\">-Megjegz\\u00e9s: <strong>pr\\u00f3ba megjegyz\\u00e9s<\\/strong><\\/span><\\/p>\\n<p><strong>\\u00dcdv\\u00f6zlettel,<\\/strong><\\/p>\\n<p><strong>Dr. Kov\\u00e1cs Norbert<\\/strong><\\/p>\\n<p><strong>------------------------------------------------------<\\/strong><\\/p>\\n<p><strong>Ez egy automata lev\\u00e9l. K\\u00e9rem ne v\\u00e1laszoljon r\\u00e1!<\\/strong><\\/p>\\n<p><strong>------------------------------------------------------<\\/strong><\\/p>\\n<p><em>A vizsg\\u00e1lati id\\u0151pont foglal\\u00e1s\\u00e1t az al\\u00e1bbi linken meger\\u0151s\\u00edtheti:<\\/em><\\/p>\\n<p><strong>Meger\\u0151s\\u00edt\\u00e9s: <\\/strong><a href=\'https:\\/\\/pecs.neurology.hu?_ea-action=confirm&_ea-app=42&_ea-t=fe2307b9981a0f978a16c03df7fbc17e\'>https:\\/\\/pecs.neurology.hu?_ea-action=confirm&_ea-app=42&_ea-t=fe2307b9981a0f978a16c03df7fbc17e<\\/a><\\/p>\\n<p><em>Amennyiben a vizsg\\u00e1lati id\\u0151pontot le szeretn\\u00e9 mondani, az al\\u00e1bbi linkre kattintson:<\\/em><\\/p>\\n<p><strong>T\\u00f6rl\\u00e9s: <\\/strong><a href=\'https:\\/\\/pecs.neurology.hu?_ea-action=cancel&_ea-app=42&_ea-t=d66a86c1153b88637b6ee4cbe68eb590\'>https:\\/\\/pecs.neurology.hu?_ea-action=cancel&_ea-app=42&_ea-t=d66a86c1153b88637b6ee4cbe68eb590<\\/a><\\/p>\",\"headers\":[],\"attachments\":[],\"phpmailer_exception_code\":2}}---------- ERROR #43 END ----------

Hi, from error log looks like there is no email custom field? Best regards, Nikola

Further to my response about also having this issue, I would like to roll back to version 1.11.7   Where can I obtain this?