How to Keep a Business Mailbox Under Its Storage Quota

A mailbox near its quota stops receiving mail, and the bounce goes to the customer, not to you. The fix is not deleting old threads; it is removing the handful of large attachments that account for most of the space. This guide shows how to find them, what to clean first, and the habits that keep a mailbox healthy for years.

Quick summary

• In webmail or your desktop app, search with a size filter: larger:10M in Gmail-style search, "size" column sorting in Outlook and Thunderbird (add the Size column to the message list and click it), or the Attachments filter in Apple Mail's search.
• Look in Sent too. Everything you attached is stored twice: once in your Sent folder and once in the recipient's mailbox.
• Save the attachments you need to your file storage (a shared drive under a client or project folder), then delete the messages or strip the attachments. Thunderbird can detach attachments to disk while keeping the message; Outlook can remove an attachment from a message with Save As and then Remove Attachment.
• Trash and Junk: empty them. Many hosts do not count them for long, but some do until you empty them.
• Sent: messages with attachments older than a year can usually go once the file is saved elsewhere.

Know your numbers

Check the quota and usage in webmail; most show a bar or a figure in Settings. OquMail mailboxes get 5 GB each on the free plan, which is years of ordinary business mail. A mailbox that fills faster is almost always attachment-driven: photos, PDFs, design files and video. Ten 20 MB attachments cost more space than ten thousand plain messages.

Step 1: find the big messages

1. In webmail or your desktop app, search with a size filter: larger:10M in Gmail-style search, "size" column sorting in Outlook and Thunderbird (add the Size column to the message list and click it), or the Attachments filter in Apple Mail's search.
2. Look in Sent too. Everything you attached is stored twice: once in your Sent folder and once in the recipient's mailbox.
3. Save the attachments you need to your file storage (a shared drive under a client or project folder), then delete the messages or strip the attachments. Thunderbird can detach attachments to disk while keeping the message; Outlook can remove an attachment from a message with Save As and then Remove Attachment.

Step 2: clean the folders that quietly grow

• Trash and Junk: empty them. Many hosts do not count them for long, but some do until you empty them.
• Sent: messages with attachments older than a year can usually go once the file is saved elsewhere.
• Notifications and alerts: anything older than 90 days from automated senders. A filter can delete these on a schedule.
• Newsletters: delete everything older than a month; the content is still on the sender's website.
• Drafts: old unsent drafts with attachments add up.

Step 3: archive by year

For mail you must keep but rarely read, move whole years into a local archive. In Outlook, File, Open and Export, Export to a file (.pst) with a date filter; in Thunderbird, ImportExportTools NG exports folders to mbox. Store the file with your business records and back it up. The messages disappear from the server quota but remain searchable in the desktop app when you open the archive file. Keep the current year plus one on the server so search from your phone still finds recent work.

Habits that keep it under control

• Send links, not attachments, for anything over a few megabytes. Both mailboxes stay small and the file stays current.
• Give automated senders their own mailbox (alerts@ or noreply@) with an aggressive cleanup rule, so their volume never touches personal quota.
• Set a quarterly reminder: sort by size, deal with the top twenty messages, empty Trash.
• Create separate mailboxes for separate functions rather than one overloaded inbox; a support@ mailbox has its own quota, and on OquMail each of the 15 free mailboxes gets its own 5 GB.

When the mailbox is already full

1. Sign in to webmail (it works even when the mailbox cannot receive) and empty Trash and Junk first. That alone often restores a few hundred megabytes.
2. Sort Sent by size and delete the largest twenty messages.
3. Test with a message from an outside account; delivery resumes as soon as usage drops below quota.
4. Then do the proper cleanup above so it does not recur next month.

Common questions

Does deleting a message on my phone free space on the server?

On IMAP, yes, once it is also removed from Trash. On POP accounts, deleting locally does nothing to the server; check the account's "leave messages on server" setting.

Will archiving to a local file lose the mail if my laptop dies?

Yes, if it is the only copy. Store the archive file on a backed-up drive or cloud storage. Treat it like any other business record.

What uses more space, many small messages or a few big ones?

A plain email is around 10 to 50 KB. A 20 MB attachment equals roughly a thousand of them. Chase the attachments first; deleting small messages rarely moves the number.