Premail.dev is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
Premail itself is licensed under the GNU Affero General Public License v3.0 or later. To be clear: This license only covers Premail, not any content you generate using Premail. Those files are yours to do with as you wish, and you need not share them, much less license them, in any particular way.
For the legal details of the AGPL, see the complete LICENSE.