Please note that this is a Joomla! support request that has nothing to do with our software. I will point you to the right direction and close the ticket. If you need more information please consult Joomla!'s documentation, the official Joomla! forum or the Joomla! Facebook group.
You have made a change in your Global Configuration, telling Joomla! to not send any email. You can change that by going to System, Global Configuration, Server tab and setting "Send Mail" to Yes (it is currently set to No).
The JLIB_MAIL_FUNCTION_OFFLINE message is the result of Joomla! not loading its base translation file before it generates this message. Unfortunately, since this message comes very early in the boot process of Joomla! you can't do anything about it :( The JLIB_MAIL_FUNCTION_OFFLINE language string reads: "The mail function has been temporarily disabled on this site, please try again later". I believe that had you seen this much more descriptive message you wouldn't have asked us what to do. Yeah, this untranslated Joomla! message got me too the first time I saw it :(
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!