Your Mailbox is Full, you may not be able to send or receive items

Dear Marvin Marvin,

Good day!

Thank you for posting to Microsoft Community. We are glad to assist.

If your email is stored on an email server such as Exchange, when your mailbox reaches its upper limit your administrator might start limiting functionality. Here are some ways to keep your mailbox size under control:

1.
The first thing you can do to reduce your mailbox size is to archive your Outbox older items. 
Archive older items – Move old items you want to keep to an archive. You can create a separate Outlook Data File (.pst) that you can open from Outlook any time you need it. By default, Outlook automatically archives items at a regular interval. You can also archive items manually whenever you want. That way, you can decide which items to archive, when to archive them, and where to store them.

2.
Empty the Deleted Items folder and Junk Email folder "Files">" Options">" Advanced" tab.

Empty the Junk Email folder – Periodically empty the Junk Email folder to save your inbox space for messages you actually want.

3.
Store attachments outside your mailbox – As attachments accumulate over time, they can take up a lot of space. For attachments you want to keep, consider saving them to a team site, to OneDrive, or to folders on your computer.

In Outlook, click on the attachment you want to save outside your mailbox.

Select the "Save As" option and choose the location on your hard disk where you want to save the attachment.

4.
Delete E-mail and AttachmentsOpen Outlook. Go to the Folder list, select "Search folders" and then choose "New Search folder".

 Scroll down the list and choose "Large mail" under 'Organizing Mail". Then, click "Choose" under "Customize Search Folder".

Specify size in the "Show mail greater than" box and hit "Ok".

Click on "Ok" again and you will a folder is created with the name "Large Than "Size" is created under "Search Folders".

When you encounter a mailbox full error problem, open the Search Folders and select the newly created folder and get rid of your mails that are not important.

5.
Use Outlook Cleanup Tools Feature

You can use Outlook Cleanup tools features to reduce mailbox size.

Go to the "File">" Info">" Cleanup Tools">" Mailbox Cleanup" in Outlook.

Choose "View Mailbox Size", Find items older than", "Find items large than", "View Deleted items size", "Empty Deleted Items" folder", "Empty Conflicts", or "View Conflicts Size" to execute your desired operation.

Manage my mailbox size - Microsoft Support
If you need further clarification, feel free to reply to this post.

Your understanding and cooperation are greatly appreciated.
wish you all the best.

Sincerely,

Winter| Microsoft Community Moderator