New answers tagged postfix
I suggest you to check this http://stackoverflow.com/questions/18318789/linux-postfix-dovecot-554-relay-access-denied smtpd_recipient_restrictions = permit_mynetworks permit_sasl_authenticated reject_unauth_destination
Another problem might be that the client you are using is not setting the Message-ID header. To fix that with dovecot use: always_add_missing_headers=yes Funny thing is, in my case it was the GMail mobile client for android which has this behaviour and failed to send mails to *@gmail.com
Another solution is to use the recipient_canonical_maps configuration parameter of postfix. Compared to the virtual_alias_maps, the advantage is that myorigin neither needs to be in mydestination nor in virtual_mailbox_maps (which virtual_alias_domains defaults to). Thus, this postfix instance does not become the final destination for myorigin. Add this to ...
While it is unsure what exactly you want to do, I can suggest this article as a good start for completely setting up IMAP/SMTP, then, setting up mail client on other hosts should be piece of cake: https://www.digitalocean.com/community/tutorials/how-to-configure-a-mail-server-using-postfix-dovecot-mysql-and-spamassasin
Multi-instance is not equal changing the number of threads. The term multi-instance is used to have multiple distinct instances running. If you wanna change the number of threads allowed to spawn for different subsystems of postfix, maybe have a look at the performance tuning? http://www.postfix.org/TUNING_README.html#proc_limit
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