is mail box exceeding 18.25 GB limit . to verify it open the mail app . select inbox & please right click on it . then click on get account info - then on summary.
if they are exceeding delete unwanted mails in inbox / send to trash .
also delete large messages with attachments . so how to verify it : close all applications by using key board short cut : command + tab + Q . then click command + F . a window will be opened click sort by at the top of the message list & choose size from pop up menu .
choose view > message attributes > size .
delete attachments that are saved . choose messages > remove attachments but the message remains in the mail box & mail indicates attachments were manually removed .
also choose mail box > erase deleted items then choose to remove from all accounts or a specific account or from mac
in the end restart from apple logo & empty the trash .
by deleting caches contents from system library & caches , cookies , saved application state contents from user library somewhat increase the hard disk space .
now, how to verify it . open disk utility directly from spotlight : click on mac HD - then click on others - some figure will be there for example : 8.3 GB note it down please . if one has deleted the caches , cookies contents that i have mentioned respectively in system library & user library . then again go to disk utility in others the figure has been reduced .
multiple photographs , songs in iTunes , lot of junk .dmg files in download folder is one of the reason to occupy hard disk space .