fix(GODT-1623): Throttle SMTP failed requests

If a SMPT client keeps hammering bridge and triggers multiple successive
errors in quick succession, force that client to wait 20 seconds before
trying again.

internal/services/smtp/errors.go

@@ -22,3 +22,4 @@ import "errors"
 var ErrInvalidRecipient = errors.New("invalid recipient")
 var ErrInvalidReturnPath = errors.New("invalid return path")
 var ErrNoSuchUser = errors.New("no such user")
+var ErrTooManyErrors = errors.New("too many failed requests, please try again later")