Email Automation, multiple inboxes. How send simultaneous process?

I am running an email automation system with a switch connecting to 11 inboxes.

Each inbox has a timer, so only an email is sent every 5 mins.

How do I get all inboxes to start processing at the same time?

Currently all emails connecting to inbox 1 must finish before inbox 2 can start and so forth.

is there a way for simultaneous process after the switch?