When you say the sending IP address is your website server address, are you referring to your external web IP, or your internal (lan) IP address ?  

How many emails are you talking about having to send ?

What are you using to mass send ? Are you sending from within smarter mail, or have you made a script to read from a database and send ? (script directly or call SM as an API ?) Or using a 3rd party product ?

I was asking about this  https://portal.smartertools.com/community/a90921/api-can-i-send-emails-via-api-insteald-os-webforms-.aspx

As we run into the same thing sort of, where a domain will kill off email addresses and our database is then filled with email addresses that no longer exist on our own servers.  While, we have a script that generates the email, I was plannning to add in some type of server connection thing to see if we can validate an email address before it is sent. this can be done with the API from what i am seeing.

In our script i had to write a timer to slow down the outbound emails in order to avoid being flagged as a spammer on our own servers and by external servers.  Depending on the domains, i have a differnet time interval and number of emails per minute i send.  I also have a flag in the database that can be used to override the domain. For instance, a lot of non profits (.org and .edu) now use google as their email backend.  Sending too many emails to google, even if they are differnet domains and not gmail accounts, can still get us flagged as a spammer. If i discover a domain email is hosted by google i set the flag as a google flag, and it slows down the outbound email even more for those accounts.