Solve 554 Email Errors with cPanel/Exim Expertise

cPanel / Exim Email Deliverability Expert Needed – 554 Errors Description: We are looking for an experienced cPanel/Exim email server specialist to troubleshoot an inbound email delivery issue on our company domain. Our company emails are working normally and we can receive emails from most external senders. However, emails from one specific government organization are being rejected or delayed by our mail server. The sender is using Proofpoint, and they are receiving errors such as: * 554 5.1.2 * 554 5.4.0 Our current setup: * cPanel hosting * Exim mail server * MX points to our cPanel mail server * Email Routing: Local Mail Exchanger * cPanel Email Deliverability: Valid * SPF, DKIM and DMARC configured * Other external emails are being received normally We have also noticed another issue that may or may not be related to the email delivery problem. Some of our employees are able to use their company email accounts normally through Outlook on their laptops without a VPN. However, for two users , the company email does not connect/sync properly unless a VPN is enabled. For example, on an iPad or MacBook, the mail server and company domain cannot be resolved/accessed normally without VPN, while enabling the VPN allows the email account to connect and sync immediately. We have already confirmed that the mail server IP itself is reachable without VPN, which may indicate a DNS resolution, network routing, hostname, SSL, or device/network-specific issue. We would therefore like the specialist to investigate both issues: 1. Why incoming emails from Proofpoint are being rejected or delayed with SMTP errors 554 5.1.2 and 554 5.4.0. 2. Why certain users/devices require a VPN to connect to our company mail server, while other employees using Outlook can access the same company email system normally without VPN. Please investigate whether these two issues are related or independent and identify the root cause before making any major DNS or mail-server configuration changes. We need someone who can investigate the server-side SMTP/Exim logs, DNS/MX configuration, firewall/anti-spam rules, greylisting, sender/recipient verification and identify exactly why emails from this specific sender are being rejected. Please only apply if you have strong experience troubleshooting cPanel/Exim SMTP 554 errors and inbound email deliverability issues.