Why is one user receiving email bounces on their PC but not their mobile device?

Key findings

• Environment Differences: The mobile device likely uses a different network connection (cellular data) or a simpler email client compared to the PC, which might be on a corporate network or running various browser extensions and local software.
• Client-Side Interference: Software on the PC, such as browser plugins (e.g., Grammarly), antivirus programs, or firewalls, can subtly alter email content or block outbound connections, leading to bounces. This is a common cause for issues with desktop clients, as discussed in our guide on fixing Outlook desktop client bounces.
• Network Restrictions: Local network settings or firewalls on the PC's connection (like a home or corporate Wi-Fi) could be blocking the outgoing SMTP traffic or routing it through an IP that is blocklisted or has a poor reputation.
• Content Modification: Even a simple "Hi" email might be altered by a PC application, injecting hidden characters or HTML that triggers spam filters, as noted by Frankenstein Computers in their article on email working on mobile but not computer. This can lead to bounces even when the core message seems innocuous. Our guide on invalid user bounces delves deeper into bounce causes.

Key considerations

• Isolate the Network: Have the user connect their PC to the same network as their mobile device (e.g., by tethering the laptop to the phone's hotspot) and try sending the email again. This helps rule out network-specific blocklists or firewalls affecting the PC's usual connection.
• Check PC Software: Investigate any browser extensions, email client plugins, antivirus software, or firewalls on the PC that might be interfering with email sending. Temporarily disabling them can help identify the culprit.
• Examine Bounce Messages: A detailed analysis of the bounce message can often provide specific error codes or reasons for rejection, pointing directly to the problem source, whether it's content, authentication, or recipient server policies.
• Test Alternate Content: While the user states simple content, sometimes even simple text can be flagged if altered or if the sending domain is new to the recipient's system. Try sending a more "normal-looking" email with a few paragraphs of text.

Key opinions

• IP/Network Blockage: A primary suspicion is that the IP address of the PC (or its network) might be blocklisted or facing temporary network restrictions that the mobile connection avoids. This type of block can lead to emails going to spam or even being rejected outright, as discussed in our guide on what happens when your IP gets blocklisted.
• PC-Specific Software: Browser plugins, email client add-ons, or security software on the PC could be interfering with the email's composition or transmission, adding hidden elements or disrupting the sending process.
• Domain Reputation Implications: While the domain is custom and not new, certain sending methods or client configurations might still impact how the domain's reputation is perceived by the recipient server, potentially causing bounces related to domain reputation, which can also lead to emails going to spam.
• Content Filtering Sensitivity: Even simple content might be suspicious if the sending context (like a specific IP or mail client) is unusual or flagged. Experimenting with more varied content is advised.

Key considerations

• Network Connectivity Test: The user should try sending an email from their PC while connected to their mobile hotspot. This helps determine if the issue is with the PC's primary network connection (e.g., corporate Wi-Fi) or the PC itself.
• Plugin and Extension Review: A thorough check of all browser extensions and email client plugins on the PC is necessary. Even seemingly innocuous ones (like grammar checkers) can inject code or text that triggers spam filters, as discussed in the Spiceworks Community.
• Firewall and Whitelisting Check: Confirm that no local firewall settings or network whitelisting rules on the PC or its current network are preventing outbound email traffic for that specific user, while other users or devices might have different configurations.
• Client-Server Dialogue Differences: Although both use the Gmail web app, subtle differences in how a desktop browser interacts versus a mobile browser/app might exist, potentially causing the PC's connection to be seen differently by the mail server.

Key opinions

• Header and MIME Differences: Experts suggest that different email clients or sending methods (PC vs. mobile) might produce subtle variations in email headers or MIME structure. These differences, even minor, can be interpreted differently by recipient mail servers, potentially leading to rejection or classification as spam.
• Authentication Inconsistencies: While core SPF or DKIM might be set up, specific client configurations on the PC could impact how authentication is handled, particularly with services like Microsoft. Issues such as SPF DNS timeouts with Microsoft highlight the complexity of authentication.
• Local System Environment: The PC's operating system, network drivers, installed software, or even malware can influence how email data is prepared and sent, potentially corrupting it or making it appear suspicious.
• IP Reputation Granularity: Even if the user moves between Wi-Fi networks, the specific outbound IP used by the PC at a given moment could be on a blocklist (or blacklist) or have a lower reputation than the mobile carrier's IPs, triggering server-side rejections as outlined by Mailgun's blog on email failures.

Key considerations

• Raw Header Comparison: Obtain the raw email headers from both a successfully sent mobile email and a bounced PC email. Comparing these in detail can reveal subtle differences in how the message is constructed and authenticated, pointing to the cause of the bounce. Understanding DMARC, SPF, and DKIM is essential for this analysis.
• Network Packet Analysis: For advanced troubleshooting, performing a packet capture on the PC while attempting to send can show if the email is even leaving the machine correctly and how it's being handled by the local network or firewalls.
• Clean Environment Test: Try sending from the user's PC in a stripped-down environment, such as Safe Mode with Networking, or a freshly installed virtual machine. This helps determine if conflicts with existing software are the root cause.
• Recipient Server Logs: If possible, reviewing the recipient mail server's logs for the bounced email attempt can provide explicit reasons for rejection from their perspective, which is often the most definitive piece of information.

Key findings

• Common Bounce Reasons: Documentation typically classifies bounces as hard (permanent, e.g., invalid address) or soft (temporary, e.g., full inbox). The specific bounce message is crucial for diagnosis, as outlined by Fastmail's help center.
• Sender Reputation Impact: A poor sender reputation can lead to emails being rejected, even for valid addresses. While the overall domain might be good, a specific IP or sending method from the PC might be flagged, affecting deliverability and inbox placement, as detailed in our article on technical solutions for email deliverability.
• Client Configuration Errors: Incorrect SMTP settings, authentication failures, or local firewall configurations on the sending client are frequently cited reasons for emails failing to send. Mobile apps often have simpler, more automated setups that reduce these errors.
• Content Filtering: Receiving servers analyze email content for spam characteristics. If the PC's client adds unforeseen elements or formatting, it could trigger these filters, leading to rejections, as highlighted in ActiveCampaign's glossary on bounced email.

Key considerations

• Review Email Client Setup: Thoroughly check the email account settings on the PC's client, ensuring SMTP server, port, and authentication methods match the provider's recommended configuration. This is crucial for addressing deliverability issues.
• Understand Bounce Codes: Consult official documentation or RFCs (Requests for Comments) to decipher specific bounce error codes. These codes provide precise reasons for delivery failure, such as 550 (permanent failure) or 421 (temporary issue).
• Network Troubleshooting: Ensure that the PC's network (Wi-Fi, Ethernet) allows outbound connections on the necessary email ports (e.g., 587 for authenticated SMTP). Corporate networks often have stricter firewall rules that might affect desktop clients but not mobile ones.
• Software Conflicts: Review any recently installed software or updates on the PC that could conflict with the email client or network drivers, leading to sending problems unique to that device.