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From historical reasons we have now the company mail server hosted on a serverA along with our dns entries and our main domain. Soon we're about to open online shop on serverB hosted elsewhere. The configuration has to stay this way.

What is the best option to send emails to shop customers (from serverB) keeping in mind that we want to use our main domain (which is hosted on serverA).

To my knowledge I see 2 possibilities:

  1. we use serverA to send emails to customers leaving smtp server on serverB - it seems unnecessary to put another smtp server in chain and it will create some delays
  2. we configure SPF dns entries on serverA to authorize serverB to send emails in behalf of our main domain - this seems the best but it's a new topic for me and I'm not sure if this gonna work as I think

Could you recommend an architecture for such a scenario?

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Both setup's are equally valid:

  • Use your existing mail server, set up a service account for your online shop to use for SMTP authentication and as long as it gets formatted correctly all mail sent from the online shop should be indistinguishable from your current email messages.

You will benefit from your existing reputation and won't have any issues with delivery issues while building up reputation from a new IP-address or ip-address range black-listings caused by other customers at your web hosting provider.
And you won't have to do anything when you scale up, move hosting providers etc.
The risk is that there is some latency between your web server and mail server.

  • Use your web server to send e-mail directly.

This has the advantage that your online shop won't have any external dependancies.

The disadvantage is that it requires you to set up a complete mail server, which is not very difficult, but may be a bit tricky.
Unless your SPF record already allows any A record to send mail on behalf of your domain, you will need to white-list your new ip-address there.
You wil probably need to set up new DKIM selector for this new server.
While your new mailserver should be configured to allow/show @example.com as the sender domain, it takes a little bit of care to ensure it won't accept @example.com addresses for local delivery, when your online shop sends messages to your own staff those should arrive in their current mailboxes and not on (non-existing) mailboxes on your web server.
You may benefit from any email reputation tied to your domain name but you might suffer from IP-address blacklist listing caused by previous users of your ip-address and/or the other customers in your ISP's ip-address range.

  • A third option may be to use an external provider to send transactional/promotional email messages from your online shop.

Many companies use a separate domain (not a sub domain of the current domain) if there is a risk that the domain will get bad reputation/ sender score.

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There is the third variation:

You can use subdomain of your primary organization domain. Create new MX record for domain shop.domain.tld (as example), where domain.tld - are company domain.

Benefits: this third-level domain will not affect to your company domain and simultaneously will contain a domain.tld suffix.

Be sure you create ptr record of your shop server's ip (most email servers checks ptr existance) and proper spf record.

  • Is this subdomain really separated from the antispam filters perspective? In other words - whether reputation of domain of higher level can influence somehow subdomain or other way around? – pMM Oct 12 '17 at 20:39
  • @pMM, anti spam technologies rely on emails content analyzation, filtering by ip addresses with ip list of antispam providers and other methods. Antispam protection does not applicable for domains because sender address can be substituted. For domain checking are used antifishing methods such as spf checking (e.g. you can set up certain ip addresses wich can send emails from your domain and set up reject policy) – Egor Vasilyev Oct 13 '17 at 7:09

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