Q: mail app disconnecting on IMAP

Hi,

Try the troubleshooting steps here >  Use Mail Connection Doctor - Apple Support

Generally SSL( secured shell layer ) is to be checked , for secure connection. But i do not know what are your port settings , is it a gmail account , yahoo or a private network .if it a private network ask port settings from your service provider .

suggested article : TCP and UDP ports used by Apple software products - Apple Support

When you click on connection doctor a red dot will appear in front of affected account .

  Please open system preferences and click on internet accounts and select the account ( gmail account )

A window will appear please enter your web mail id and correct password . it will take some time to configure .

Open connection doctor all accounts must have green dots in front of them .

if still , it shows connecting , i would recommend to take back up of all mails and delete the IMAP account and once again fresh account is to be configured .