Gmail has recently introduced a new restriction on Gmail, that limits your mail space once you‘ve reached certain storage, which means until you declutter your old emails, you won’t be able to receive new ones. 

So, how to overcome this problem when there's no "mass delete" button on Gmail?  Easy. You do the following trick: 

1. First, open Gmail and type is:read in the search bar

2. Click the Select All Conversations button, as shown below. Do note that this feature will only pop up if you have more than 100 read messages. 

3. Select the trash icon and watch your space declutter itself. Important: this will delete all your read emails, so if you have any important emails that you'd rather keep, we suggest putting them in another folder before proceeding. 

Depending on how many read emails you have accumulated, the deleting process will take some time. However, this trick is only applicable in the web version of Gmail, so attempting this using phone would not work. Nonetheless, that's just a minor inconvenience compared to a cleaner, clutter-free Gmail account, yes?