Outlook Lesson 7: Archiving and Mailbox Size

📎 This article includes 1 downloadable practice file ↓

⏱ 3 min read

📘 Outlook Beginner Course · Lesson 7 of 8

In this article
  1. Check the size
  2. Find what’s big
  3. Shrink it
  4. Archive folder vs online archive
  5. PST files
  6. Retention policies
  7. Where beginners go wrong
  8. Practice

“Your mailbox is almost full” usually arrives at the worst moment, and when it’s full you can’t send mail. Knowing what fills a mailbox and how to trim it keeps you out of trouble.

Check the size

Classic Outlook: File › Tools › Mailbox Cleanup › View Mailbox Size, or the bar under your account on File › Info. New Outlook / web: Settings › Accounts › Storage.

Find what’s big

  • Search size:>5MB (or the Search ribbon › Size › Huge).
  • Mailbox Cleanup › Find items larger than 5,000 KB.
  • Sort the Sent Items folder by size: your own big attachments are often the worst offenders.

Shrink it

  1. Save important attachments to OneDrive or a project folder, then delete the email or just the attachment (classic Outlook: open the email, right-click the attachment › Remove).
  2. Empty Deleted Items and Junk; they count towards your quota.
  3. Clean Up Conversation on long threads (Lesson 1).
  4. Next time share OneDrive links instead of attaching files.
💡 Sending a file as a OneDrive link takes almost no mailbox space for you or the recipient, and everyone sees the latest version.

Archive folder vs online archive

  • The Archive folder (Backspace/Archive button) just moves mail out of the inbox; it’s still in your mailbox and counts towards size.
  • An Online Archive (In-Place Archive), if your organisation enables it, is a separate mailbox with its own large quota. Retention policies can move mail older than, say, two years there automatically.

PST files

Classic Outlook can export mail to a .pst file on your PC (File › Open & Export). It frees space, but PSTs live only on that computer: lost laptop, lost mail, and they aren’t searchable from your phone. Many companies block them. Prefer the online archive.

⚠️ Never ‘archive’ by forwarding mail to a personal Gmail account. It breaks most company policies and can leak customer data.

Retention policies

Your organisation may delete or archive mail automatically after a period. Look for retention labels on folders (right-click › Assign Policy) and ask IT what applies before relying on old mail being there.

Where beginners go wrong

Habit Fix
Never emptying Deleted Items Empty it weekly, or auto-empty on exit
Attaching 20 MB files repeatedly Share links
PST files as the only backup Online archive or OneDrive

Menus differ slightly between classic Outlook for Windows, the new Outlook and Outlook on the web; where they differ, both are given.

Practice

Download the hands-on checklist below and do the Lesson 7 section in your own Outlook. Nothing in it sends mail to anyone else.

📎 Practice files for this article

  • 📘
    Outlook hands-on checklistStep-by-step tasks for all 8 lessons to do in your own Outlook.
    ⬇ DOCX · 37 KB

Free to use for learning. Files with macros (.bas) are plain text — import them with Alt+F11 → File → Import File, and always test on a copy.

Keep reading

OutlookOutlook Lesson 6: Signatures and TemplatesOutlookOutlook Lesson 8: Shared Mailboxes and DelegationOutlookOutlook Lesson 1: Folders, Focused Inbox and a Clean Inbox
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