Why are email attachments being lost when sending from Java API to G Suite accounts?

Key findings

• Recipient-side filtering: G Suite domains may employ custom email routing or third-party filtering solutions that can strip attachments if they are deemed suspicious, even if G Suite's default behavior is not to strip them. These services often operate beyond your direct control.
• Attachment characteristics: Specific file types (like PDFs or XMLs), their internal structure, or even seemingly small file sizes (e.g., 51KB) can be flagged if they contain elements that resemble malware or are unusually formatted. For more about email filtering, consider why your emails are going to spam.
• Java API implementation: Errors in how the Java API encodes and attaches files, particularly for binary data like PDFs, can lead to the attachment being corrupted or unreadable by the recipient's mail client. This often relates to MIME type handling.
• Authentication standards: While SPF and DKIM might be set up, the absence of a strong DMARC policy or issues with DMARC alignment can sometimes contribute to stricter filtering, although it's less direct with attachment stripping. Ensuring proper DMARC, SPF, and DKIM is always a best practice.
• Normal Gmail behavior: Gmail processes millions of attachments hourly without issues, suggesting that the problem is specific to the sending method, attachment content, or particular recipient settings, rather than a general Gmail limitation.

Key considerations

• Examine raw email source: After sending, verify the raw MIME structure of the sent email to ensure the attachment (PDF, XML) is correctly encoded and included before it leaves your system.
• Test with various recipients: Conduct tests to different G Suite domains, including those without custom filtering, to isolate whether the issue is widespread or isolated to specific recipients.
• Recipient communication: Engage with the affected recipients to understand their email security configurations and if they receive notifications when attachments are blocked. This can provide direct clues.
• Security scan attachments: Use a robust antivirus or security scanner on the specific PDF and XML files to rule out any embedded malicious code or suspicious elements that could trigger filtering.
• Review API documentation: Consult the Gmail API documentation or Java Mail API best practices for sending attachments, especially for binary types, to ensure correct implementation.

Key opinions

• Inbox settings are primary culprits: Many marketers suspect recipient inbox settings, antivirus software, or other third-party services as the main reason attachments go missing, especially when core email infrastructure (like G Suite) is not known for stripping them. This is often the case when emails are frequently going to spam in Gmail or G Suite.
• Domain-specific filtering: If recipient domains run their own mail transfer agents (MTAs) with additional filtering, they are more likely to strip attachments that appear 'iffy' or unknown, regardless of sender reputation.
• Attachment content scrutiny: Marketers frequently question if something within the specific PDF or XML files makes them look potentially malicious, as Gmail generally handles millions of attachments without issue.
• Limited whitelisting options: While it's difficult to get whitelisted by large providers like Google, individual users might be able to whitelist specific senders to prevent attachment stripping.
• API implementation is key: Validating that the mail sending code in Java is functioning as intended and correctly sending all attachments is a crucial first step in troubleshooting, as suggested in discussions on Java Mail API.

Key considerations

• Verify outgoing attachments: Before troubleshooting recipient issues, confirm that the attachments are actually leaving your sending system or Java application correctly. Use logs or a testing tool.
• Check for recipient notifications: Ask recipients if they receive any bounce messages or notifications when attachments are blocked, as this can indicate a specific policy or filter at play.
• Test with plain text attachments: Try sending a simple email with a plain text attachment using the same Java API setup to see if it delivers successfully, which can help narrow down the problem to file type specific issues.
• Isolate problematic accounts: If the issue is only with a few accounts, focus troubleshooting on those specific recipients' email configurations, as it suggests the problem isn't a systemic G Suite issue. This is similar to troubleshooting suddenly rejected by Gmail scenarios.

Key opinions

• MIME structure integrity: Experts stress that proper MIME encoding and multipart message construction within the Java API are critical for attachment delivery. Any malformation can lead to attachments being unreadable or stripped. This can also cause RFC 5322 compliance errors.
• Content inspection by security gateways: Advanced email security gateways (e.g., antimalware, DLP solutions) can perform deep content inspection of attachments, even small ones, and might block them if they detect suspicious patterns or known vulnerabilities within the file format.
• Sender reputation's role: A poor sender reputation, even with proper authentication, can lead to stricter filtering rules applied by receiving mail servers, increasing the likelihood of attachments being dropped or flagged as spam. Understanding your email domain reputation is vital.
• Raw log analysis: Analyzing mail server logs at both the sending and receiving ends (if possible) provides concrete evidence of where the attachment is being dropped or if any explicit rejections are occurring.
• Base64 encoding: For binary attachments (like PDFs), ensuring correct Base64 encoding is crucial for safe transmission across different email systems. Improper encoding can lead to corruption or stripping. For more details, consult Medium's article on Gmail API PDF issues.

Key considerations

• Inspect generated files: Have a security expert review the generated PDF and XML files for any code or structural anomalies that could trigger security filters, even if they appear benign to the sender.
• Verify Java Mail API configuration: Ensure that the Java Mail API or custom Java code is correctly setting content types (e.g., application/pdf, application/xml) and handling the attachment's input stream without corruption.
• Monitor DMARC reports: While not directly reporting attachment issues, DMARC aggregate reports can provide insights into overall email authentication failures or rejections by G Suite, which might correlate with stricter content filtering. Consider understanding DMARC reports.
• Test with different email clients: Have recipients check their emails on different clients (web, desktop, mobile) to see if the attachment is visible in some but not others, which might indicate a client-side rendering issue rather than a full strip.

Key findings

• Base64 encoding requirement: Documentation often specifies that binary attachments like PDFs must be encoded in Base64 format to be safely transmitted within an email body, ensuring data integrity across various email systems.
• API for sending attachments: APIs like the Gmail API are designed to handle email sending, including attachments, by allowing programmatic access to mailboxes. This suggests that if attachments are missing, the issue is likely not a fundamental API limitation but rather how it's implemented or how the receiving server processes the mail.
• Error handling mechanisms: Documentation often includes guides on handling API errors, such as refreshing access tokens for authentication issues. While this doesn't directly address attachment stripping, it highlights the importance of stable API connections for reliable email delivery.
• SMTP size limitations: Standard SMTP servers impose maximum attachment sizes for performance reasons. Although 51KB is small, extremely restrictive or misconfigured servers could theoretically still enforce a lower limit that causes issues, leading to emails being rejected by Gmail.

Key considerations

• Adhere to MIME standards: Ensure that your Java API implementation strictly follows MIME (Multipurpose Internet Mail Extensions) standards for structuring emails with attachments, including proper content-type headers and boundaries.
• Check content-disposition: Verify that the Content-Disposition header for attachments is correctly set, typically to 'attachment' with a filename, to ensure mail clients recognize them as downloadable files.
• Review API library usage: If using a client library, ensure it automatically handles token refresh and other common issues. For manual API calls, follow error handling guidelines provided by Google.
• Consider character sets: For XML attachments, verify the character set encoding used in the file and ensure it is consistent with the email's declared character set to prevent parsing errors by the recipient's system.