Emails taking longer to send

Hello,

Emails are now taking longer to send than they used to before, they queue  up for users and eventually send bit by bit eventually getting sent 3-6 hrs later. Please add to my list of areas / questions to help with this troubleshooting?

To start with, first thing would be to check.

1. Have outgoing emails incremented in the System than before? 
2. Are users now sending a higher volume than before?

3 .... What else

I don't have a lot of knowledge about emails and email routers.

Thanks,