Download Old Messages in Airmail is a feature that allows you to determine the volume of messages in relation to the time period that you want to keep in Airmail. This option is useful if you want to save the disk space and do not want the complete database to be downloaded locally from the servers.
To find this option, go to 'Airmail Preferences > [Account] > More' and here you will be able to choose the option in 'Download Old Messages'. Please refer to the picture below:
Once you choose the limit from the available options, proceed on to rebuild the account database by selecting the option for 'Clean Messages Cache'. In the example, the value for the 'Download Old Messages' is set to '1 Month' and then follow as shown in the picture below:
Please note: Airmail will add the new emails to this newly cleaned database. For example in the case above the database will be '1 month + The new mails'
Also, this feature is not available for Exchange and POP accounts.