[Feature] IPv6 support

[Joseph Allan]

Currently smtp4dev listens only on IPv4. Would it be possible to enable IPv6 support?

Possibly either dual stack by default and/or configurable between:

• Dual stack
• IPv4 only
• IPv6 only

Very much appreciate all your work on this excellent tool!

[Rob Wood]

Implemented in PR #1408.

Dual stack is on by default. See appsettings "disableipv6"