The "Backup SMTP" function in MailDroid allows you to create backup SMTP providers for your email account. There are many cases which this can be helpful, such as a corporate account that allows sending only on their wifi or a case where once connected to your home wifi you must use a secondary smtp provider. Here is the simple workflow:

You can add as many SMTP backup providers as you like and MD will try them in order. You can remove any provider by clicking the - button next to the provider.

In the future, more options may be added to this feature.