postconf -e soft_bounce=yes
postconf -e smtp_fallback_relay=otherhost:port
postconf -e bounce_queue_lifetime=0
soft_bounce (default: no)
Safety net to keep mail queued that would otherwise be returned to the
sender. This parameter disables locally-generated bounces, changes the
handling of negative responses from remote servers, content filters or
plugins, and prevents the Postfix SMTP server from rejecting mail
permanently by changing 5xx reply codes into 4xx. However, soft_bounce
is no cure for address rewriting mistakes or mail routing mistakes.
Note: "soft_bounce = yes" is in some cases implemented by modifying
server responses. Therefore, the response that Postfix logs may differ
from the response that Postfix actually sends or receives.
soft_bounce = yes
bounce_queue_lifetime (default: 5d)
Consider a bounce message as undeliverable, when delivery fails with a
temporary error, and the time in the queue has reached the
bounce_queue_lifetime limit. By default, this limit is the same as for
Time units: s (seconds), m (minutes), h (hours), d (days), w (weeks).
The default time unit is d (days).
Specify 0 when mail delivery should be tried only once.
This feature is available in Postfix 2.1 and later.
smtp_fallback_relay (default: $fallback_relay) Optional list of relay
hosts for SMTP destinations that can't be found or that are
unreachable. With Postfix 2.2 and earlier this parameter is called
fallback_relay. By default, mail is returned to the sender when a
destination is not found, and delivery is deferred when a destination
is unreachable. With bulk email deliveries, it can be beneficial to
run the fallback relay MTA on the same host, so that it can reuse the
sender IP address. This speeds up deliveries that are delayed by
IP-based reputation systems (greylist, etc.). The fallback relays must
be SMTP destinations. Specify a domain, host, host:port, [host]:port,
[address] or [address]:port; the form [host] turns off MX lookups. If
you specify multiple SMTP destinations, Postfix will try them in the
specified order. To prevent mailer loops between MX hosts and
fall-back hosts, Postfix version 2.2 and later will not use the
fallback relays for destinations that it is MX host for (assuming DNS
lookup is turned on).