SMTP Proxy, no internal mailserver, want to use the smtp proxy to scan outgoing mail from different users on gmx.net. how to configure that

Question

Hi there, 
 
 a customer of mine has just a few email adresses on gmx. no mailserver internally, but has an utm. every outlook uses pop to get their mails on their own. no popcon or anything. 
 pop proxy is set up and works fine. now they want to use the smtp proxy as well to check their outgoing mail. how do i need to configure the smtp proxy? if using the smarthost section under advanced i can only use one user to authenticate at the gmx smtp server. which means for that user the proxy works fine. the others bounce though. 
 
 any help would be appreciated 
 
 thx in advance 
 
 Alex

apijnappels · Answer

You cannot do it with the gmx.net mail addresses. gmx.net has an SPF record ending in -all which means they are specifically denying any other ip-address than the ones specified in the SPF record to send mail on behalf of gmx.net. 
 That means that when you start using Sophos UTM as the SMTP proxy (it can do that and send mail out to the big world all by itself without a smart host), a lot of sent mails will get listed as spam simply because of this spf setting on gmx.net. 
 The only thing you can do is start using their own domain and start using a mailserver somewhere (Office365 is not that expensive, but there are several others providers too that give mailboxes with just POP/IMAP access with custom domains. Of course having a mailserver internal is also an option, but might be more costly to maintain with just a few mailboxes.