We are receiving emails but can't send when using Verizon for ISP

Hello, I work for a company that uses Comcast as a secondary ISP and we also have Verizon Fios for our primary ISP.  Two days ago we started noticing that our outbound emails weren't being sent through outlook.  This is accross 6 different @comcast.net emails.  I check to see if I could do it on the xfinity portal online but it sent fine on there.  Went to outlook to make sure settings were all good and they are.  Nothing has changed other than not being able to send emails out.  The error we receive when trying to send is 

"Task 'commerciallaundry@comcast.net - Sending' reported error (0x800CCC67) : The server responded: 421 resomta-ch2-05v.sys.comcast.net resomta-ch2-05v.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not available'

This is weird because everything was working just fine and no password has been changed.  So I dug around and someone mentioned something about it being a Verizon ISP issue and how Comcast was blocking the IP from the Verizon router/modem.  So since we have Comcast as well I decided to try and test this.  

All I did was change the network from using Verizon router to using the Comcast router and POOF it worked. No settings changed or anything.  SO I thought it was odd, took my iphone out, that is connected to FIOS wirless, and tried to add the email account to test it from there, but it came back with an error about the SMTP server will not connect.  I switch off FIOS to my 4G network and POOF it works. 

So I've narrowed it down to something about using Verizon for the ISP... tried everything else.  Called support about 5 times, they all just pass the blame from Microsoft, to Verizon to finally the last guy just said IDK sorry and that was it.  So I need to get this fixed.  What can I do?