Timeouts, for server and client.

[novedevo]

per RFC:

 An SMTP client MUST provide a timeout mechanism.  It MUST use per-
 command timeouts rather than somehow trying to time the entire mail
 transaction.  Timeouts SHOULD be easily reconfigurable, preferably
 without recompiling the SMTP code.  To implement this, a timer is set
 for each SMTP command and for each buffer of the data transfer.  The
 latter means that the overall timeout is inherently proportional to
 the size of the message.