Use IPs as LMTP addresses

Currently, remote LMTP servers are only addressable via hostname. It would be easier if IP addresses could also be used as remote LMTP addresses.

Current config
---
```
[remote."lmtp"]
address = "mylmtpserver"
port = 11200
protocol = "lmtp"
...
```

Desired config
---
```
[remote."lmtp"]
address = "10.1.0.2"
port = 11200
protocol = "lmtp"
...
```