outlook storage

Question

I want to know, how we store emails and attachments in outlook. Just like gmail uses base64. what are the things we use to compress the emals and attachments in outlook. explain with example if 2 mb pdf file is stored in outlook, after compressing what will it show. explain this for 2Mb pdf file

2Mb jpg file

2Mb png file

2mb docx file

2mb xslx file

2 mb txt file

2 mb csv file

2 mb ics file

Answer 1

Hi Pragati Pandey

From what I research, Outlook Classic does not store emails or attachments using Base64 encoding. Messages are stored using the native MAPI format, and attachments are saved as raw binary data. There is no automatic compression or optimization applied based on file type. As a result, a 2 MB attachment (PDF, JPG, PNG, DOCX, XLSX, TXT, CSV, or ICS) occupies approximately 2 MB in the mailbox.

Base64 encoding is used only when an email is sent via SMTP, typically to external recipients. During the sending process it increases the attachment size by approximately 33%, so a 2 MB file becomes roughly 2.6–2.7 MB while in transit. Message size limits are enforced on this encoded size, not the stored mailbox size. This behavior is consistent across all attachment types.

Read here for more details: Exchange Online limits - Service Descriptions | Microsoft Learn

However, if a file is attached as a OneDrive link, the file itself is not attached in the message and is not Base64‑encoded, which avoids attachment size inflation.

I hope this information helps.

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