See https://discussions.apple.com/thread/8087473 for infos.

This java programm will manipulate your Apple Mail Account in a way to allow insecure authentication. Furtheremore, there is a crude PHP tool that does the same.

0. Setup an account in Mail up to the point where you need to tick the allow insecure flag and save
1. pkill Mail
2. open with a SQLite Browser (such as sqlitebrowser.org) ~/Library/Accounts/Accounts4.sqlite
3. From ZACCOUNT table find the account you want to fix and note the Z_PK and ZACCOUNTTYPE of it
4. From the ZACCOUNTTYPE table get all rows that have ZKEY = PortNumber, DisableDynamicConfiguration or AllowsInsecureAuthentication and where ZWONER is Z_PK of ZACCOUNT
5. Insert into ZACCOUNTPROPERTY Z_ENT=3,Z_OPT=1,ZOWNER=Z_PK of ZACCOUNT, ZKEY = ortNumber, DisableDynamicConfiguration or AllowsInsecureAuthentication, ZVALUE a blob containing the value (see examples)
6. Start Mail