Domain Email Bouncing "Mailbox Full" – Fix in 3 Steps

Before You Start – The 30-Second Fix

I've seen this a hundred times. Someone's email starts bouncing back with "Mailbox full" and they panic. But half the time, the fix takes thirty seconds. Here's the first thing to check.

1. Check the Outbox – Open Outlook (or whatever client you're using). Look at the Outbox folder. If you've got a 500MB video attachment sitting there, that's your problem. Delete it. Send a test email. If it goes through, you're done.
2. Check the Sent Items – Scroll through your Sent Items folder. Do you have 1,000 emails from the last week? Each one counts toward your mailbox size. Delete the ones you don't need.

After doing this, wait 5 minutes and try sending an email again. If the bounce stops, stop here. If not, move to the next step.

Real-world example: A client at a law firm had this issue last month. Turns out they had a 2GB PowerPoint deck stuck in the Outbox for three days. Deleted it, problem solved.

Step 2 – The 5-Minute Fix: Clear Deleted Items and Recoverable Items

If the quick check didn't work, the real culprit is likely the Deleted Items folder or—on Exchange servers—the Recoverable Items folder. Most people don't realize that deleting an email doesn't actually free up space. It just moves it to a hidden folder that still counts toward your mailbox limit.

Clear Deleted Items in Outlook

1. Right-click the Deleted Items folder in Outlook.
2. Choose Empty Folder.
3. Yes, you're sure.

Clear Recoverable Items (for Exchange/Office 365)

This is the one people miss. Exchange has a hidden folder called Recoverable Items that holds deleted emails for 14-30 days. It counts toward your mailbox quota.

1. Open Outlook on the web (not the desktop app).
2. Right-click Deleted Items and select Recover deleted items.
3. You'll see a list of emails that are already "deleted" but still taking up space. Select them all (Ctrl+A).
4. Click Purge. This permanently removes them and frees the space.

After purging, close Outlook, reopen it, and try sending a test email. Should go through now. If not, move to Step 3.

Step 3 – The 15+ Minute Fix: Increase Your Mailbox Quota or Clean Deeply

This is the advanced fix. If the first two steps didn't work, either your mailbox quota is set too low for your usage, or you've got a hidden monster taking up space.

Check Current Mailbox Size

First, let's see how big your mailbox actually is.

• Outlook desktop: File > Account Settings > Account Settings > double-click your email account. Look for "Mailbox Size" or click "More Settings" > "General" tab > "Mailbox Size". Note the total size.
• Outlook on the web: Settings (gear icon) > View all Outlook settings > General > Storage. It'll show you the total used space.

If you're over 90% of your quota, that's why bounces happen. Exchange servers typically set quotas at 50GB, but your admin might have set it lower (like 5GB or 10GB).

Option A: Increase the Quota (If You're the Admin)

If you have Exchange admin rights, you can bump up the limit. Here's how to check and change it.

For Exchange On-Premises (Exchange 2013/2016/2019):

1. Open the Exchange Management Shell as administrator.
2. Run this command to see the current limits for a user:

Get-Mailbox -Identity "user@domain.com" | Format-List ProhibitSendReceiveQuota, ProhibitSendQuota, IssueWarningQuota

This shows you the three mailbox limits. If they're all set to "unlimited" but you're still getting bounces, skip to Option B below. Otherwise, you can increase them:

Set-Mailbox -Identity "user@domain.com" -ProhibitSendReceiveQuota 100GB -ProhibitSendQuota 95GB -IssueWarningQuota 90GB

Then run the Get-Mailbox command again to verify the changes took effect.

For Office 365 / Exchange Online:

1. Go to the Exchange Admin Center (admin.exchange.microsoft.com).
2. Go to Recipients > Mailboxes.
3. Select the user, click the pencil icon, then Mailbox usage.
4. Under Storage limits, you can set custom values (e.g., 100GB for the Prohibit Send limit).
5. Click Save.

Option B: Find and Delete Large Items (If You Can't Increase Quota)

If you're not the admin or the quota is already maxed out, you need to clean house. Use this Outlook method to find the space hogs.

1. In Outlook, go to your Inbox.
2. Click the Search box at the top.
3. Type: size:>10485760 (that's 10MB in bytes). Press Enter.
4. This shows all emails over 10MB. Sort by size (click the Size column header). Delete the biggest ones, especially those with large attachments.
5. Repeat this search in Sent Items and any other folders.

You can also target specific sizes: size:>100000000 for 100MB items. I've found that users often have one massive email—like a 200MB PDF—lurking in their Sent Items that they forgot about.

Option C: The Nuclear Option – Archive or Export to PST

If you're still stuck and can't increase the quota, move old emails to a PST file. This is a last resort because it means those emails are no longer searchable on the server, but it'll fix the bounce issue fast.

1. In Outlook, go to File > Open & Export > Import/Export.
2. Choose Export to a file, then Outlook Data File (.pst).
3. Select the folder you want to archive (like Inbox or Sent Items).
4. Check the box Do not export deleted items (saves space).
5. Choose a location to save the PST file (like an external drive).
6. Wait for the export to finish. Then delete the older emails from the server (move them to Deleted Items and empty it).

After archiving, your mailbox size will drop. Try sending an email again.

What If None of These Work?

If you've done all three steps and emails are still bouncing, the problem might not be a full mailbox at all. Check these two things:

• Check the exact bounce code. If it says "550 5.1.1 Mailbox full" but you have plenty of space, your server might be misconfigured. Contact your hosting provider or Exchange admin.
• Check if the recipient mailbox is full. If you're sending TO someone and getting the bounce, their mailbox might be full. This happens when you email an external domain (like a client). Ask them to check their mailbox size.

I've included the most common fixes here, based on real tickets I've handled. Start with Step 1, and only go deeper if you have to. Most people stop at Step 2 and it works.