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The Natural Philosopher  wrote:
> As part of my enhancements to the home controller, I want to send email 
> warnings of low heating oil level.
> 
> So I removed exim which was too much like hard work, installed PostFix 
> and all is well...,except it isn't.
> 
> If I send a mail out of the blue after a a few hours delay, the mail 
> immediately *always* gets stuck in the queue with a message about being 
> unable to determine the IP address of the SMTP relay.
> 
> If I send another message everything Just Works, and indeed the h mail 
> queue gets flushed eventually as well with a successful delivery.

'Greylisting'?  SMTP servers often delay mail from unrecognised senders as a
spam block - spammers tend not come back when told to try again later.

Also the message needs to be correct to match SFP/DKIM/DMARC rules,
otherwise it may get delayed or discarded.  It may also be delayed/discarded
due to sending from a 'consumer' IP.

Perhaps by the time the second message arrives the spamfilter has decided
you're ok and to let the message straight through.

I would not be doing direct SMTP in this day and age - send using an SMTP
account (with username/password) at the company who runs the SMTP server for
the sender's mail domain.

If the SMTP relay is indeed your domain's mail server, perhaps the first
time DNS lookup is too slow?

Theo

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