Sending email - Our issues and how we fixed it.

[Frank Cheale]

I have seen a number of issues concerning email failing so I thought I'd share our experience of how we have implemented outbound email. Hopefully someone may find this helpful.
Issue: We are using AWS. By default AWS blocks sending mail on port 25, the barrier to overcome this block for our limited use case seemed very onerous. We discounted the AWS email solution as too complex for our limited needs.
Hence the need to use an SMTP relay agent. Whilst it is possible to use Hotmail or Gmail as the relay agent it was found emails regularly got blocked by the intended recipient as spam. This was mostly blocked at the mailing system level and the intended recipient would often not be aware or see any email in their spam folder.
Using a “trusted” SMTP relay for sending emails was the best solution for us. We chose the service mailersend - https://www.mailersend.com/

mailersend required that the originating email address matched to our pre-configured trusted domain name. We found that a high percentage of email was failing because the originating email address was not using our host details but the email address of the user sending the email. To overcome this issue we had to configure postfix to change the sending email address. The  two links below give detail of how to change postfix configuration.
https://unix.stackexchange.com/questions/379175/how-do-i-change-postfix-sender-address
https://serverfault.com/questions/147921/forcing-the-from-address-when-postfix-relays-over-smtp
Alternatively consult the postfix manual.

ATB
Frank