If your server has multiple IP addresses/interfaces available then you can provide a comma separated list of these IP addresses for EmailEngine to bound to when making outbound connections.
This is mostly useful if you are making a large amount of connections and might get rate limited by destination server based on your IP address. Using multiple local addresses allows to distribute separate connections between separate IP addresses. An address is selected randomly from the list whenever making a new IMAP connection.
$ emailengine --service.localAddresses="192.168.1.176,192.168.1.177,192.168.1.178"
If those interfaces aren't actually available then TCP connections will fail, so check the logs.
Local addresses and SMTP
By default when EmailEngine is sending an email to SMTP it uses local hostname in the SMTP greeting. This hostname is resolved by
os.hostname(). Sometimes hostname is using invalid format (eg.
Servername_local as undersore is not actually allowed) and depending on the SMTP MSA server it might reject such connection.
To overcome you can set the local hostname to use by appending the hostname to the IP address, separated by pipe symbol
$ emailengine --service.localAddresses="ip1|hostname1,ip2|hostname2,ip3|hostname3"
For example when using AWS you can use the private interface IP but set a public hostname.
$ emailengine --service.localAddresses="172.31.1.2|ec2-18-194-1-2.eu-central-1.compute.amazonaws.com"
So in general the hostname shoud be whatever the public interface IP (this is what the SMTP MSA server sees) resolves to.