[SLUG] postfix error sending email every 15 mins

[Ben Donohue <donohueb@xxxxxxxxxxxx>]

Hi all,

I have a postfix email server with dovecot.
Someone I sent an email to with an attachment is getting the email every 
15 minutes.
I presume it's my mail server resending it as there was some error 
somewhere.
After Googling a bit I've run...

postfix flush

I'm not a postfix guru so is there any other way to check if this has 
been stopped?
Where can I look to see the queue?
Any other tips?

Thanks
Ben