Hi bKumanski,
Thanks for posting in Apple Support Communities. We noticed that you recently reviewed Manage your iCloud storage which is the best resource to help you make storage space. You mentioned deleting emails and items remaining in trash. You should be able to empty the trash as well if needed. Check out the following section from the article above.
"iPhone, iPad, or iPod touch
If your iPhone, iPad, or iPod touch is set up to access iCloud email, you can delete messages from any mailbox, then empty the Trash to free up space:
Swipe left across any message to delete it.
Go to your Mailboxes and choose your Trash folder.
To delete messages one at a time, swipe left on each message. To delete everything, tap Edit, tap Select All, and then tap Delete. Tap Delete All to confirm.
Mac or Windows PC
If you use iCloud Mail with a desktop email app, such as Mail on your Mac or Microsoft Outlook on your PC, use these steps:
Mail on your Mac: Delete the messages that you no longer want, then choose Mailbox > Erase Deleted Items and select your iCloud email account from the menu. Then click Erase to confirm.
Microsoft Outlook 2010 through Outlook 2016 (Windows): Delete the messages that you no longer want. Outlook will permanently delete your messages the next time you close and restart the app.
iCloud.com
Sign in to iCloud.com.
Select Mail and choose your Trash folder.
To delete messages one at a time, select the message you want to delete, then select the Trash button . To delete multiple messages, select the messages you want to delete then select Delete."
All the best.