Why is one user receiving email bounces on their PC but not their mobile device?
Michael Ko
Co-founder & CEO, Suped
Published 21 Jul 2025
Updated 16 Aug 2025
9 min read
It can be incredibly frustrating to troubleshoot email bounces, especially when the issue appears inconsistently across different devices for the same user. I often encounter scenarios where a user's emails bounce when sent from their PC but go through without a hitch from their mobile device, even to the exact same recipient. This behavior immediately suggests that the problem isn't solely with the recipient's mailbox or the fundamental email server configuration.
The key here is to look beyond the obvious. While the email content and recipient address remain constant, the underlying environment from which the email is initiated changes. This includes factors like the network connection, browser-specific elements, local software, and how the email client on each device processes and sends the message. We'll explore these nuances to pinpoint why such a discrepancy might occur and how to resolve it.
The sending environment
The sending environment plays a crucial role in how an email is constructed and transmitted, influencing whether it's accepted or rejected by recipient servers.
Browser extensions and hidden content
Even when using a web-based email client like Gmail through a PC's web browser, extensions can significantly alter the email's underlying HTML. Tools like Grammarly or tracking plugins can inject hidden characters, formatting, or code into the message body that isn't visible to the user. While seemingly innocuous, these hidden elements can sometimes trigger spam filters on the recipient's server, leading to a bounce (or blocklist listing).
The disparity between PC and mobile device in this scenario might arise because mobile apps or mobile browser versions typically don't support the same range of extensions, or they process emails in a cleaner, more standardized format. If the PC's browser is adding problematic hidden content, this could be the sole reason for the bounce. It's a subtle but common issue that can be tricky to diagnose without examining the email's raw source.
Network differences and IP reputation
Another primary suspect is the network connection itself. When a user sends an email from their PC, it typically uses the IP address of the local Wi-Fi or wired network. A mobile device, however, might use cellular data or a different Wi-Fi network, resulting in a completely different sending IP. If the PC's network IP address is on an email blacklist or blocklist, it can prevent emails from being delivered, even if the sending domain's reputation is otherwise good. Understanding how email blacklists actually work can shed more light on this.
Even if the user travels and uses various Wi-Fi networks, it's possible one or more of these networks' IPs could be flagged. This situation highlights why it's critical to consider the sending IP's reputation, distinct from the domain's reputation. A quick test involves connecting the PC to the mobile device's hotspot. If emails send successfully then, it strongly indicates an issue with the PC's primary network IP being on a blocklist (or blacklist).
Discrepancies in email processing
While we've ruled out common issues like a new domain or cold outreach, deeper technical factors related to how the email is formed and authenticated can still cause bounces.
Content scrutiny beyond the visible text
Even a simple email containing just "Hi" can trigger filters if the email's underlying structure, headers, or metadata are unusual. Different devices and email clients, even web-based ones, can format emails in subtle ways. A PC's browser might introduce specific HTML quirks, character encoding issues, or even hidden formatting from a copy-paste operation that a mobile client does not. Recipient servers are highly sensitive to anything that deviates from expected email standards. Most bounced or rejected email includes an error message explaining why your email did not reach your recipient. Pay close attention to these details.
This leads me to consider the specific bounce message the user is receiving. Bounce messages often contain error codes (e.g., 550, 554) and accompanying text that can provide invaluable clues. For instance, a message related to content or spam filtering would point towards the email's body, while a message about a Sender Policy Framework, or SPF error, would suggest an authentication issue related to the sending domain. Even if SPF is configured, subtle differences in how the PC sends it versus the mobile can cause this.
Local client settings and caching
While the user is sending from the Gmail web application, the specific browser, its cache, cookies, or even browser-level security settings on the PC could differ from the mobile experience. A corrupted browser cache, outdated browser version, or an overly aggressive firewall/antivirus on the PC might be interfering with the email's transmission or content generation, leading to discrepancies.
Furthermore, if the user had previously configured a desktop email client (e.g., Microsoft Outlook) on the PC and then switched to the web app, residual configurations or system-level settings could still be affecting email sending. Sometimes, troubleshooting email bounces when sending to Outlook desktop clients requires examining the specific client's configuration.
Sender authentication and reputation
While the immediate thought might be content or IP issues, the underlying domain reputation and authentication mechanisms are always at play, even if not directly causing the bounce in all contexts.
Domain reputation and authentication
Even though the user confirmed SPF and domain registration are fine and it's not a new domain, the overall email domain reputation is critical. Email Service Providers (ESPs) and Mailbox Providers (MBPs) use sophisticated algorithms to evaluate sender trustworthiness. This evaluation considers the sending IP, the sending domain, and the content itself. A bounce message often speaks to the return-path domain's reputation or, in rarer cases, problematic body content.
It's possible that the mobile device's sending method, perhaps through a different mail server or API endpoint for the same service, is slightly different in how it handles authentication headers, like DMARC, SPF, and DKIM. If the PC's path introduces a subtle misconfiguration or anomaly in these headers, it could cause recipient servers to reject the email, even if the primary SPF record itself is valid. This is especially relevant if invalid user bounces beyond IP reputation are occurring.
Advanced troubleshooting
When the obvious solutions fail, it's time to consider less common but equally impactful causes for email bounces. These often involve deeper system-level interactions or dynamic filtering mechanisms.
Malware and system integrity
A concerning possibility is that the user's PC is infected with malware. Malicious software can subtly alter outgoing email messages, injecting spam triggers, malicious links, or otherwise modifying the email's content or headers in a way that causes it to be rejected. Even if the visible message content is just "Hi," the unseen alterations could be what's leading to the bounces from the PC, while the mobile device remains unaffected.
It's essential to perform thorough malware scans and ensure the operating system and all software are up to date. Security software on the PC might also be overly aggressive, interfering with legitimate email sending. A comprehensive system check should be part of the troubleshooting process for persistent, device-specific bounce issues.
Temporary server issues or dynamic filtering
While less likely to be consistently device-specific, temporary server issues at the recipient's end or dynamic filtering based on traffic patterns could contribute. Sometimes, recipient mail servers implement real-time (or near real-time) filtering that can temporarily block certain sending IPs or content patterns if they detect unusual activity. A mobile device sending from a different IP or through a slightly different route might bypass these transient blocks.
Views from the trenches
Best practices
Always inspect the full bounce message for error codes and descriptive text, as it contains precise clues from the recipient server.
Test sending the same email from the problematic PC using a different network, like a mobile hotspot, to isolate network-related issues.
Regularly review and disable non-essential browser extensions that interact with email composition, as they can inject hidden content.
Perform comprehensive malware scans and system updates on the affected PC to rule out system compromises or software conflicts.
Verify the email's raw source (headers and body) from both PC and mobile if possible to spot subtle differences in formatting or content.
Common pitfalls
Assuming the problem is with the recipient's server without considering local PC configurations or network-specific blocklists.
Overlooking hidden HTML or text injected by browser extensions, which can trigger spam filters without being visible.
Failing to differentiate between the sending IP's reputation and the sending domain's overall reputation.
Not considering that even simple content like 'Hi' can be flagged if the sending infrastructure (IP, authentication) is compromised or misconfigured.
Neglecting to check for local firewall rules or antivirus software that might be interfering with outgoing email traffic from the PC.
Expert tips
When a bounce points to domain reputation, investigate the return-path domain, as it's often more scrutinized than the 'From' domain by receiving servers.
For content testing, use a lengthy, neutral text snippet (like a Wikipedia article paragraph) instead of short, common phrases that might be over-filtered by new or zealous spam rules.
If the user's account or domain is relatively new, some mailbox providers might apply stricter filtering until sufficient positive sending history is established.
Consider that a client's machine might be infected, causing emails to be subtly altered and leading to bounces, even if the user is unaware.
Remember that mobile email apps often use different underlying APIs or routing paths compared to web browsers, which can bypass PC-specific issues.
Marketer view
Marketer from Email Geeks says the PC's IP might be blocked and suggests trying to connect the PC to mobile Wi-Fi to test the theory.
2022-07-22 - Email Geeks
Expert view
Expert from Email Geeks says a plugin like Grammarly on the PC but not the mobile could be injecting hidden text into the HTML body, causing the issue.
2022-07-22 - Email Geeks
Solving the device-specific email bounce puzzle
The perplexing scenario of a single user receiving email bounces on their PC but not their mobile device, despite using the same email service, underscores the complexity of email deliverability. It rarely points to a single, simple cause. Instead, it typically involves a combination of factors related to the local sending environment.
By systematically investigating network IPs, browser extensions, hidden content, local system integrity, and subtle authentication path differences, you can effectively diagnose and resolve these elusive email bounce issues. Patience and methodical troubleshooting are key to getting those emails delivered reliably, regardless of the device.
Gmail through a PC's web browser, extensions can significantly alter the email's underlying HTML. Tools like Grammarly or tracking plugins can inject hidden characters, formatting, or code into the message body that isn't visible to the user. While seemingly innocuous, these hidden elements can sometimes trigger spam filters on the recipient's server, leading to a bounce (or blocklist listing).
Microsoft Outlook) on the PC and then switched to the web app, residual configurations or system-level settings could still be affecting email sending. Sometimes, troubleshooting email bounces when sending to Outlook desktop clients requires examining the specific client's configuration.
