MX priority order is not respected

[richfelker]

MX records are processed in the order DNS returns rather than according to priority. This may prevent successful delivery with some receiving domain setups. Results should be sorted by priority, and there might should be treatment of early errors (e.g. in initial greeting) as cause to try the next MX rather than a temp failure to report via exit status (RFC is ambiguous on requirements here and existing mailers differ).

[richfelker]

Fixed by 64838ee. There's still no fallback except on connection failure but this should be ok.