Fix SMTP on magento 2.3.3
Please unzip file to folder contains magento code
Then run command
patch -p1 < EmailMessageInterface_2.3.3_backward_compatibility_composer-2019-10-30-10-20-09.patch
https://magento.com/tech-resources/download#download2333
EmailMessageInterface_2.3.3_backward_compatibility_composer-2019-10-30-10-20-09.patch.zip