Your message is too large for the receiving mail system to process, even if the individual mailbox could accept it. This is a system-wide size limit, not a per-mailbox limit. Reduce the message size.
What Does Error 5.3.4 Mean?
Enhanced status code 5.3.4 indicates the message exceeds a system-level size limit on the receiving server. This is different from 5.2.3 (mailbox-level limit) because it is the entire mail system that cannot handle the message size, not just the individual recipient's quota.
This limit is typically set by the mail server administrator and may be lower than what individual mailboxes could theoretically accept. Some organizations set conservative limits (5-10MB) for all incoming mail regardless of individual mailbox capacity.
Common Causes
- Message exceeds system-wide size limit (including attachments and encoding)
- Organization has set conservative system-level size limits
- Attachments after Base64 encoding exceed the maximum
How to Fix Error 5.3.4
- Reduce attachment sizes or use cloud file sharing links
- Compress files before attaching
- Check if the recipient organization has published size limits
- Strip unnecessary embedded images from HTML email
Frequently Asked Questions
Bounce code 5.3.4 means your email (including all attachments) exceeds the maximum message size allowed by the recipient's mail system. This is a system-wide limit that applies to all mailboxes on that server. It is a permanent failure -- the message cannot be delivered at its current size. Common limits range from 10 MB to 50 MB depending on the email provider, with Gmail allowing up to 25 MB.
Reduce your email size by compressing attachments, using cloud storage links (Google Drive, Dropbox, OneDrive) instead of direct file attachments, or removing unnecessary images and embedded content. Remember that base64 MIME encoding increases attachment size by approximately 37%, so a 20 MB file becomes about 27 MB in transit. Split large files across multiple emails if cloud storage is not an option.
Gmail allows up to 25 MB for both incoming and outgoing messages. Microsoft Outlook/Exchange Online has a default limit of 25 MB but can be configured up to 150 MB by administrators. Yahoo Mail allows up to 25 MB per message. These limits apply to the total encoded message size (body plus all attachments after MIME encoding), which is larger than the raw file sizes. For files exceeding these limits, use cloud storage sharing links instead.
Your email's actual transmission size is larger than the raw attachment file size due to MIME/base64 encoding, which increases the size by approximately 33-37%. A 20 MB file becomes roughly 27 MB after encoding. Additionally, the email body, headers, HTML formatting, embedded images, and signatures all contribute to the total size. To stay within limits, keep your total pre-encoding attachment size at about 70% of the server's maximum limit.