I have configured a ShrePoint 2013 on-premise which i would like to send Mails via Exchange to external Users. I can receive Mails from MySites internally but when I choose an externally adrees like @gmail.com I get "Sorry, you are not allowed to share this with external users." I've looked around where I've to change this in Central Administration but I can't find anything.
This is limitation of SharePoint workflow engine 2013, it can't send email to external users.
Alternatively, you could try to use workflow engine 2010 or 3rd party products.
If you consider 3rd party products, I would like to recommend you try to use Workflow Actions Pack and "Send E-Mail with Attachments (SMTP)" workflow action (because I'm one of developer of this product and I proud for it)