Hi @BULBOUS Environmental & Logistics ,
Welcome to our forum!
What's your account? Exchange online or exchange On-premises? Or something else?
Please help us by advising that how we can increase storage or make a room to send and receive emails.
For increasing storage for your mailbox, it depends on your account and what subscription you have. If you are using Exchange online account, please refer to the below image about the mailbox size for different types of licenses and the following article: Increase or customize Exchange Online mailbox size.

For making a room, agree with @Matthew Bechtol ,we can move old items you want to keep to an archive, a separate Outlook Data File (.pst) that you can open from Outlook any time you need it. More information: Archive items manually.
Hope the above help!
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