Email Storage Limits
What a Mailbox Quota Covers
An email storage limit is the maximum amount of data a mailbox can hold, usually set by the mail host as a quota. The quota covers the inbox, sent items and other folders, and attachments count toward it. Some systems also count deleted items until they are emptied, so a mailbox can fill up even when the inbox looks short.
Mail is stored on the server's disk, so every mailbox uses part of the host's storage. Quotas stop one account from using space that other accounts need. Limits differ between providers and plans, so check the figure for your own account rather than assuming a standard value. The storage behind an account is covered in email hosting.
Message Size Limits and Attachment Growth
A message size limit is different from a quota. It caps the total size of a single message, including its attachments, that a server will accept. A message over the limit is refused, and the sender receives a bounce explaining that it is too large. Sending and receiving servers may each set their own limit, and the smallest one applies.
Attachments also grow in transit. Email was designed for text, so binary files such as photos and PDFs are converted to a text form called base64 encoding. This conversion makes the attachment roughly a third larger than the original file. A 10 MB file therefore needs about 13 MB of space in transit, which is why a file that seems within the limit can still be refused.
When a Clinic's Mailbox Fills Up
A small clinic receives patient forms by email into one shared mailbox. The mailbox reaches its quota, and new messages begin to bounce back with a notice that the mailbox is full. Staff do not notice until a patient calls to say the form was never received. Archiving older messages, saving large attachments elsewhere and asking the host to review the quota solve the problem.
Frequently asked questions
What happens when an email mailbox is full?
Once a mailbox reaches its quota, the server usually refuses new incoming messages. The sender normally receives a bounce notice, explained in email bounces, and the message is not delivered. Some servers reject mail at once, while others ask the sending server to try again later. Freeing space or raising the quota restores normal delivery.
Why does an attachment bounce when my mailbox has room?
The message size limit is separate from the mailbox quota, and it applies to the whole message. Base64 encoding also makes an attachment larger in transit. A file that fits in your free space can still exceed the size limit. Compressing the file, resizing images or sharing a link to the file usually avoids the problem.