Technically, you can use any SMTP Service.
However, Mailgun has the deepest integration allowing you to see much deeper email deliverability data inside your Growably Dashboard.
You can find details on how to add your own Mailgun account to Growably here.
If you don’t want to use Mailgun – then some others options are:
You can even use your own Gmail or Office365 account, however be careful as these services aren’t built for mass automated email sending so you may find some limitations & restrictions.
Of course, you are welcome to use our Mailgun account as long as you are happy with the lack of control and sending limit restrictions (more details here).