Building mail reputation — so your mails actually arrive

Mail doesn’t arrive everywhere, because mail servers filter hard on the receiving side. Even if your DKIM, SPF and DMARC are set up perfectly (we do that with Mailcow), mails can land in the spam folder — if the sender reputation of your domain isn’t right.

Building reputation is a marathon, not a sprint. Here are the most important levers.

What we do on the server side

As a reminder, we take care of the technical side:

• DKIM key generated and published in DNS
• SPF record configured correctly
• DMARC with a p=quarantine policy
• MTA-STS to enforce TLS
• Reverse DNS (PTR) of the sending IP points to the domain
• IP reputation monitoring: we regularly check whether our IPs are on blacklists

That’s the technical foundation. But: reputation is built through behavior, not through configuration. And behavior = you as the sender.

Lever 1: ramp up sending volume slowly

If you’ve just set up a brand-new domain (say, after moving to us), it has no sender history yet. Receiving mail servers treat new domains with skepticism at first.

First 4 weeks: no bulk-sending campaigns (no newsletter to 1,000+ recipients in one week). At most 100–200 mails per day in total.

Weeks 5–8: increase carefully. If mails arrive without problems (low spam-folder rate at recipients), you can double the volume per week.

From month 3 onward: reputation is usually established. Bulk mails to your legitimate lists then run without problems — as long as the content isn’t spammy. For real mass sending (thousands of recipients per day), Mailhosting is not the right tool; there are specialized newsletter providers for that.

This is called warming up the domain.

Lever 2: get recipients to reply

Mail servers also rate reputation by reply rate: how often do recipients answer your mails?

• Personal mails (1-to-1) generate replies → good reputation indicator.
• Newsletters without a reply prompt → low reply rate → reputation slowly degrades.

If you send a newsletter: design it so recipients can reply (e.g. “Reply with your questions”). Even a few replies help.

Lever 3: avoid spam complaints

When recipients click the “Mark as spam” button, that feeds into the receiving mail server’s reputation monitoring. At Gmail, Microsoft 365, Yahoo: every spam complaint lowers your reputation.

Tips:

• Clear sender address: not noreply@ for personal communication. From our own experience: recipients mark mail as spam because they don’t know who you are.
• Clear subject line: no click bait, no “AMAZING OFFER!!!”.
• No advertising tone in private correspondence: distinguish between business and private communication.
• Working unsubscribe link in newsletters: prevents the “spam click” and is a GDPR requirement.

Lever 4: check authentication headers

If mail lands in a recipient’s spam folder, ask the recipient to show the mail headers and send them to you.

The header contains lines like:

Authentication-Results: mx.google.com;
       dkim=pass header.i=@deinverein.de;
       spf=pass smtp.mailfrom=deinverein.de;
       dmarc=pass action=none header.from=deinverein.de

All three at pass is the minimum requirement. If one says fail or softfail: server setup problem — open a ticket with us, we check the DNS records and fix them.

Lever 5: mail content

Spam filters also look at the content. Classic spam indicators (avoiding all of them improves reputation):

• Excessive capitals: “AMAZING!!!” → spam score goes up
• Lots of exclamation marks: one or two okay; ten definitely suspect
• Spam words: “free”, “guaranteed”, “buy now”, “safe”, “urgent” — not each word in isolation, but in combination
• HTML-only without a plain-text version: spam filters like multipart mails (HTML + plain text)
• Embedded images without alt tags: resembles spam patterns
• Shortened links (bit.ly, tinyurl): many spam filters actively block them
• Content only in images: spam filters can’t read images for content → suspect

If you send newsletters, run them through mail-tester.com — you get a 0-to-10 score plus concrete recommendations.

Lever 6: keep the bounce rate low

Bounce rate is the share of your mails that come back as undeliverable — either because the recipient address doesn’t exist (hard bounce) or the mailbox is full (soft bounce).

High bounce rates hurt reputation significantly:

• Over 5% hard bounces → receiving servers classify you as “spammy”.
• Over 10% → mails get partly blocked outright.

What to do:

• Maintain your mailing list regularly — remove non-existent addresses.
• Never send to bought lists — that’s not just a reputation risk but a GDPR risk too.
• Use double opt-in on your newsletter signup form — prevents typo addresses and signup spam.

Lever 7: check blacklist status

Sometimes a sending IP ends up on a public blacklist (Spamhaus, Barracuda, etc.). Mail servers worldwide use these lists — a listed IP is practically unable to send.

We monitor this continuously. If one of our IPs lands on a blacklist (rare, but it happens), we contact the list operators, clear up the cause and get the IP delisted.

If you want to check yourself:

• mxtoolbox.com/blacklists.aspx — enter your domain, checks 100+ blacklists.

If you spot a blacklist entry we haven’t noticed yet: open a ticket.

Lever 8: use postmaster tools

Large receivers offer postmaster tools for reputation monitoring:

• Google Postmaster Tools: postmaster.google.com — shows how Gmail rates your domain.
• Microsoft SNDS/JMRP: for Microsoft 365 and Outlook.com — shows spam reports and sender reputation.
• Yahoo Sender Hub: similar, for Yahoo Mail.

Setup per tool: 5–15 minutes. Domain verification via a DNS record.

From our experience: very useful especially for newsletter sending. You can see whether your domain is rated good, medium or bad in the spam detection of the big mail providers.

Recurring delivery problems — what we can do

If mails keep landing in spam despite all this, get in touch. We can do quite a bit on the server side:

• Rotate DKIM keys: old key compromised? A new one helps.
• Adjust the DMARC policy: switch from p=quarantine to p=reject once all tests are clean.
• IP change: in rare cases we can switch your sending IP to another of our addresses if a specific one is blocked by a recipient.
• Direct contact with the recipient’s postmaster: we contact the postmaster team at Gmail/Microsoft when legitimate mail is systematically blocked.

Frequently asked questions

How long does building reputation take? At normal volume: 4–8 weeks. With an aggressive ramp-up phase (too many mails too early): it can stretch to 3+ months, because reputation grows slowly.

Why do my mails land in spam at Gmail but not at Microsoft 365? Different mail providers have different spam filters. Gmail is more aggressive with new domains, Microsoft more tolerant. Outlook.com/Hotmail can behave differently again. Solution: build reputation specifically with the problematic provider (send to addresses there, get recipients to reply).

Can I send bulk mail from xaweho? Small, legitimate lists — a club circular, a double-opt-in list with a few hundred recipients — yes. Mass newsletters with thousands of recipients per day: no, we deliberately don’t offer that — sending like that wrecks the server reputation for all customers. There are specialized newsletter providers for that. Bought lists and spam-style sending are off-limits, full stop.

What does dkim=pass header.i=@deinverein.de mean? The mail was DKIM-signed for the domain @deinverein.de, and the receiving server verified the signature successfully. Exactly what we want.

Should I switch to p=reject in DMARC? We set defaults to p=quarantine (spam folder). reject blocks completely — stricter, but riskier if there are setup hiccups. Only switch if you’re sure and have tested all sending paths.

What if I only have one mailbox for personal mail? With pure 1-to-1 communication, building reputation is simple: write normal mails, get replies, that’s it. Problems usually only show up with bulk sending.

Does this cost extra? No. Reputation care is part of our mail server plan.

What’s next

• Understanding spam and junk
• First login as domain admin
• Changing your Mailcow plan