I think I understand what you're asking. You need to first use RequestOpenMailbox() then make your calls to SendMail(...). After you finish mailing you should call CloseMailbox().
Also note if you're sending multiple mails, you will likely need to include an escalating delay in between each sent. Something like:
Lua Code:
local delay = 0
while(stuff) do
zo_callLater(function() SendMail(...) end, delay)
delay = delay + 200
end