[Sequence diagram] Padding for outgoing allows

Question

For example, I write a sequence diagram with outgoing messaging as below:

@startuml
mainframe sd SideArrows
[->A:From leftside to A
    alt something right
        A->>]:Throwing a message to Right
    else somthing wrong
        [<<-A:Throwing a message to Left
    end
    [<--A:return
@enduml

To avoid outgoing arrows are overlaying to the alt-box, How to padding between mainframe and alt-box?