This only supports two addresses as the intended use is 1 IPv4 and 1 IPv6
address on a single-homed host, and the only supported configuration of
outgoing connections to other servers is to bind a single IPv4 or IPv6
address.
The sendq limit is now soft, now we halt processing if a sendq is exceeded, until it is sufficiently drained.
This allows us to implement SAFELIST and other floody commands without hacks.
