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

Marketer from Email Geeks suggests tethering the laptop to the phone, using the phone's service to send the email from the PC to see if the same bounce occurs.

Email marketer from Email Marketing Forum suggests testing with different email clients (e.g., Thunderbird, Outlook) on the PC to determine if the issue is specific to one client.

Email marketer from StackExchange explains that the email content itself might trigger spam filters when sent from the PC, perhaps due to specific keywords or formatting. Mobile emails might be formatted differently.

Email marketer from Email Marketing Tips Blog says that incorrect email client settings on the PC, such as SMTP server details or port numbers, could lead to sending issues. Advises verifying the settings.

Email marketer from Webmaster Forum mentions that an incorrect time and date setting on the PC can sometimes cause issues with SSL/TLS certificates, leading to email sending problems.

Email marketer from Reddit r/emailmarketing raises the possibility of malware on the user's PC injecting unwanted content or altering email settings, causing bounces. They advise running a malware scan.

Email marketer from Tech Support Forum points to possible firewall settings on the PC blocking email traffic or specific ports used for sending emails. Recommends checking the firewall configuration.

Email marketer from EmailChump Community Forum suggests that the user's PC IP address might be blocked by the recipient's email server or firewall. The different network used by the mobile device could be the reason why it's working.

Email marketer from SuperUser advises to try connecting the PC to a different network (e.g., tether to phone's mobile data) to rule out network-specific issues with the original connection. This is similar to what worked in the original question.

Expert from Spam Resource suggests that the PC's IP address might have developed a poor reputation due to past spam activity or blacklisting, leading to email bounces. The mobile device, using a different network, wouldn't be affected by this.

Expert from Word to the Wise suggests that it is possible that the computer has malware running in the background which is interfering with email sending. This could be because it has been compromised.

Expert from Email Geeks suggests to check if they are using a plugin like Grammarly on their PC and not the mobile, as it could be injecting a bunch of hidden text into the HTML body.

Expert from Email Geeks mentions that if the user's account is brand new or the domain is newly set up in Gmail, it could be overly zealous guard rails that will improve over time.

Expert from Email Geeks recommends trying alternate content, suggesting a neutral content test to rule out weird filtering of the body copy, especially if the domain is new or new to Google for Business. Also suggests sending from Hotmail to see if a reply is possible.

Expert from Email Geeks says to check for malware, disable plugins, reboot, install updates, try mailing other domains, etc.

Documentation from Apple Support recommends using Mail Connection Doctor to diagnose network and server connection problems within the Mail application on macOS.

Documentation from Postmark states the sending IP reputation of the user's PC could be negatively impacting email delivery. Whereas the mobile app may be routing email via a different mechanism that has better IP reputation.

Documentation from Gmail Help explains that browser extensions can sometimes interfere with Gmail's functionality. Recommends disabling extensions one by one to identify the problematic one.

Documentation from Microsoft Support suggests a corrupted Outlook profile on the PC as a potential cause. Recommends creating a new Outlook profile to test if the issue persists.

Documentation from cPanel says the PC might be using a different DNS server than the mobile device. The DNS server could be having issues resolving the recipient's email server address correctly.