How to connect SMTP using only IPv4

I believe IPv4/v6 is not a Laravel business but a thing that operating system should be aware of.

You can:

1. Disable IPv6 at all on operating system level (for example, in CentOS this is done by nmtui program)

2. Add to hosts file something like: 1.2.3.4 email.example.com so email.example.com will always be resolved using IPv4 address 1.2.3.4. Hosts file is inside /etc folder in Linux and in C:\Windows\system32\drivers\etc for Windows.

I'd prefer #1.

There is a third way, dirty and bad: you can set MAIL_HOST=1.2.3.4 (ip address) and disable certificate checking in mailer settings, like so:

// config/mail.php

'mailers' => [
    'smtp' => [
        // ...other settings...
        'stream' => [
                'ssl' => [
                    'allow_self_signed' => true,
                    'verify_peer' => false,
                    'verify_peer_name' => false,
                ],
            ],
            'verify_peer' => false,
    ],
],

Also, check here: https://symfony.com/doc/current/mailer.html#binding-to-ipv4-or-ipv6

Laravel uses this Symfony Mailer component for sending emails; there is an option to bind to ipv4, so maybe you can add custom option source_ip = 0.0.0.0 to your mailer config:

// config/mail.php

// ...
'smtp' => [
    // ...other options
    'source_ip' => '0.0.0.0',