I am looking for the following:

- Need a service that will queue mail in case my main email server goes down, so when it becomes available again it will deliver any queued mail
- Need to be able to deliver emails to my email server through a non-standard port

Currently I have store/forward set up and working. Will this service also retain emails (like Backup service) if my main email server goes down?

Yes it will until your SMTP orders its deletion. Can be tested by checking the Email Delivery Queue on the Dynu portal when your service is down.