Hi there. Hi there! My name is***** am a computer professional with over 15 years of experience with both Mac and Windows computers, phones, tablets, and printers. It would be my pleasure to assist you with this. Just to be sure that I understand clearly, it sounds like you had a laptop that you upgraded from Yosemite to El Capitan. Right after the upgrade you began having issues with your Mail program. Did you then purchase a new Macbook Pro and transfer your data from icloud or a time machine backup to the new computer? Then did you have the same problem on your new computer? I'm asking because I want to be sure that I clearly understand the problem. If I am understanding it correctly, then your mailbox data is corrupt and you aren't going to be able to restore it, unless possibly you restore it from an earlier backup, before the problem occurred. We can of course set Mail back up from scratch and you'll be able to use it. but depending on how you had your accounts setup it may or may not be possible to retrieve old emails. Can you tell me please first, am I am understanding this correctly, and secondly, who is your email provider? What is the last part of your email address ( the part after the @ sign)
So are you saying that if you are not connected to the work network directly or by vpn, Mail opens normally?
I am terribly sorry, but this looks as though it may take quite some time to resolve and I will have to log off shortly so I am going to opt out and open this up to other experts who may be able to help you. You won't have to repeat anything. The next expert will see our entire conversation. You'll be notified when another expert begins to work on the question. Have a great evening!
Do you know if these email accounts are IMAP accounts?
If you have already checked the drive for errors then I suggest you just delete both accounts then add them back