Just to be clear, this is a peer to peer support forum. While on occasion, an Apple tech will reply here most replies are from users just like you.
I was assuming you were a single user not from a large group like your institute. Normally, you do not install the newest version of an OS to your users since the first release is generally buggy. Someone should have installed High Sierra to test before having others upgrade.
I have two suggestions until Apple fixes.
1) Use webmail. Since it's IMAP all changes will be reflected when you download.
2) Use Outlook, Spark, Thunderbird, Postbox(has 30 day trial) or other third party app to see if they will connect.