Fixing Plesk “SMTP Ports Blocked” Issue When IPv6 Breaks Outbound Mail

[b4oshany (Oshane Bailey)]

@b4oshany

Step 4: Make the OS Resolver Prefer IPv4

The final piece of the puzzle was Plesk’s own SMTP port checker. It uses the system resolver, which was still preferring IPv6 (AAAA records).

The fix: edit /etc/gai.conf and ensure IPv4 precedence:

sudo nano /etc/gai.conf

Uncomment or add:

precedence ::ffff:0:0/96  100

This line tells getaddrinfo() to prefer IPv4 over IPv6 globally.

Short cut version:

sudo sed -i 's/^#\s*precedence ::ffff:0:0\/96  100/precedence ::ffff:0:0\/96  100/' /etc/gai.conf