I deal with it in the following way: External calls are only triggered on Commands, never on events. In order to achieve this, I use process manager. Say the

I deal with it in the following way: External calls are only triggered on Commands, never on events. In order to achieve this, I use process manager. Say the event "User Signed Up" is dispatched. A process manager will then execute the "Send Welcome Email", the aggregate checks if the user aggregate root is in a state where a welcome email should be sent (something like a flag `welome_email_sent`). Depending on the result of your email provider, the event "WelcomeEmailSuccessfullySent" or "WelcomeEmailNotSent". It is possible to react to the erroneous event with another process manager and retry.