I'm using Nintex for Sharepoint on-prem and I want to send attachments to external emails, is it possible?
Yes it is possible.
if i understood u correct u want to send a document from a library...u can set the checkbox "Anhänge" ... then it will automatically choose the element where the workflow is started from
@EhabYounis in the To field's people picker when you open a directory, you need to select the external email and type the whole email address to do so or use the single-line text column from the item property.
And as @Aleximo mentioned in the email notification you can check the option to include list attachments as an attachment or if it's the library then you can attach the file as an attachment.