Postfix issues; potential switch MTA to Exim

[mvl22]

The ongoing unreliability of Postfix suggests we may wish to switch to Exim, to give us better local support [see MB261524].

https://www.4armed.com/blog/install-and-configure-exim-4-smtp-relay-via-smarthost-on-ubuntu/

is a useful guide. The smarthost URL will need to be a configuration option obviously.

Otherwise, we need to work out why Postfix is occasionally losing its host lookup.

[nikolai-b]

Should be fixed in 9c364e6 and f4f03cc

[mvl22]

Thanks.

Can you remind me where the specific smarthost domain name we are using is defined? Presumably this should not be in the public versioning and indeed isn't in the above commits.