Sendmail problems - are they to do with DNS or not?

[Anthony Seddon]

Hi,

I am running sendmail 8.9.3 on a Linux box and the mailserver is connected
to the internet via an ISDN router, which dials up as and when needed.
Client PCs in the office use the linux box for internal and internet email.

Our domain is byzak.co.uk and we can normally send 'internal' emails without
any problems.  However, if for some reason the ISDN connection is down, then
all emails including emails to our domain are queued / deferred.  I
understand that this would be the case for emails outside our domain, but
shouldn't emails to the byzak.co.uk domain go regardless?

We are using Windows 95/98 clients running MS Outlook Express 4.  Outlook
Express doesn't complain when sending the mail, however it takes about 60
seconds to send the mail (a small text message).

Do I need to install BIND on the Linux box and set up an MX record pointing
to itself?

Am I right in thinking that sendmail does a DNS lookup on the sender?  If
that is the case, then can I disable it as by the nature of our dial on
demand connection, only people 'within the office' and within the domain can
use our server to send mail.

TIA,

Ant