Inbox placement at Proximus, Telenet, Gmail and Outlook: how to test per provider
Proximus, Telenet, Gmail and Outlook each judge the same message in their own way: the Belgian providers lean more on classic filters and reputation lists, Gmail and Outlook mainly on the behaviour of their users. A message that reaches the inbox at one can land in spam at another. The only reliable way to know is to test with a real address at each provider separately.
Why the same message is treated differently
Every provider builds its filter from different data. Gmail and Outlook see enormous volumes every day and know exactly what their users open, delete, archive and report as spam. Their filter is therefore largely behaviour-driven: how your previous messages were received weighs heavily. Belgian providers such as Proximus (with the domains proximus.be and skynet.be) and Telenet are smaller and typically combine spam filter products, third-party reputation lists and their own rules. That makes them neither stricter nor milder, just different: technical signals such as a missing DKIM record, a sending IP on a blocklist or an inconsistent reverse DNS carry relatively more weight there.
For you this means: one campaign, four outcomes. Whoever only checks their own mailbox sees just one of them.
What all providers have in common
Three things are checked by everyone. First, authentication: SPF, DKIM and DMARC must be correct, and since the large providers tightened their requirements for bulk senders, that is no longer optional. Second, the reputation of the sending domain and IP addresses. Third, list quality, visible through bounces and complaints. Whoever has those three in order has a good foundation at all four providers. The differences lie in what happens next.
Read how to set up SPF, DKIM and DMARC in a few minutes if you have not done so yet; little of what follows makes sense without it.
The typical differences per provider
Gmail
Gmail does not only sort into inbox or spam, but also into tabs such as Promotions. A newsletter in Promotions is not a delivery problem, but a visibility problem. Gmail also reacts strongly to engagement: addresses that have not opened your mail for months drag down placement at all other Gmail addresses. Gmail offers its own Postmaster tool where you can follow your domain's reputation with them.
Outlook and Microsoft 365
Microsoft uses its own reputation systems and is quicker to quietly place mail from unknown senders in the Junk folder, especially in business mailboxes where administrators add extra rules. With business recipients there is often an additional external spam filter chosen by the organisation itself. Microsoft also offers a tool to follow the reputation of your IP addresses.
Proximus and Telenet
Both are consumer providers with many older, rarely used addresses. That has two consequences. On these domains your list contains relatively many addresses that are no longer read, which lowers your engagement figures. And technical anomalies are punished faster there: a sending server without correct reverse DNS or an IP on a widely used blocklist is often refused at the handshake rather than placed in spam. You then see that as a bounce with a refusal code, not as a silent spam placement. At these providers, look at your refusals first.
How to test per provider
- Create a test address per provider. A free Gmail and Outlook address, plus a Proximus and Telenet address if you can get hold of one (a customer or colleague with such an address will gladly help). Put them in a separate test segment.
- Send the real campaign, not a test version. A test from the editor often takes a different route. You want to know how the real message, with real tracking links and the real sender, is treated.
- Look at the folder, not just arrival. Inbox, Promotions, spam, or nothing at all? Note it per provider.
- Compare with your log. If the message shows as delivered in your sending log but sits in spam, it is a placement problem. If it shows as refused, it is a technical or reputation problem.
- Repeat with every change. New domain, new template, new sender name: test again each time.
In the 2Mail dashboard you see per sending domain how deliveries, bounces and refusals are distributed. Combine that with clicks per recipient domain: a provider where delivery is normal but clicks lag noticeably is almost certainly a provider where you are in spam.
What not to do
Do not rely on a single free spam score tool that gives a mark out of ten. Those tools check technical points (useful) but cannot imitate the behaviour of the real filters. Do not send different content per provider to outsmart the filter either; that works briefly at best and makes your measurement worthless. Work on the foundation: deliverability is the same combination of authentication, reputation and list quality at all four providers.
Frequently asked questions
Which provider is the strictest?
That differs per sender and per moment. A sender with a technical problem runs into trouble sooner at Proximus or Telenet; a sender with a dormant list sooner at Gmail. Test instead of guessing.
Can I see which folder my mail ended up in?
Not from your sending log; the recipient does not report that back. That is why you need your own test addresses, and use clicks per recipient domain as an indirect measure.
Does a separate sending domain per provider help?
No. What does help is separating transactional email from campaigns, so that a poorly received newsletter does not drag your order confirmations down with it.