If you are running out of space in your Nexus mailbox (basic capacity is 2GB) then you can request an increase for free to 3GB. Additional increases are a chargeable service from IT Services but it works out quite expensive. It is £1.50 per GB per month charged twice years in arrears. So having a 5GB mailbox would cost you £36.00 per year!
One option is to clear out old large messages from your Nexus mailbox. If you have Outlook then there is a built-in search query that can show you all the large messages in your mailbox. It is located on the left hand panel underneath all your folders.
Expand the Search Folders item in the left hand panel and select Large Mail – Outlook will then gradually build up a list of large messages for you to sort out.
Open the email at the top of the listed search and right-click the email attachment and choose:
Save All Attachments…
and then Remove Attachment
Hope this helps!