Fix Microsoft 365 email delivery issues

Email that doesn’t arrive is more than annoying — it’s lost orders, missed meetings, unhappy customers and staff wasting time chasing ghosts. For UK businesses with 10–200 people, those failures ripple faster than you’d think. This guide keeps the tech jargon light and the business impact front and centre, so you can spot the root cause and decide whether to fix it yourself or get help.

Why email delivery matters to your business

Your inbox is the operational hub: invoices, supplier confirmations, job applications and legal notices. When Microsoft 365 mail gets delayed, routed to spam, or bounces, the cost isn’t just IT time. It’s delayed cashflow, reputation risk, and distracted staff. I’ve seen this in practice across SMEs from a London accounting firm to a manufacturing office in the Midlands — the symptoms are the same, and so are the outcomes.

Common causes (and what they mean for you)

1. Misconfigured DNS records (SPF, DKIM, DMARC)

These are the little postal rules that tell the internet your email is genuine. If they’re missing or set up incorrectly, big providers will label your mail as suspicious. The result: important emails diverted to spam or blocked entirely. Fixing this is straightforward but technical; if it’s mishandled it can make things worse.

2. Licensing and mailbox issues

Sometimes the reason is boring: a user’s licence lapsed, a mailbox is over quota, or a forwarding rule is misconfigured. These are quick wins — check licence status in the admin centre and confirm mailbox sizes before assuming a major outage.

3. Spam filters and transport rules

Your anti-spam settings or custom transport rules might be overzealous. That “protective” rule set up years ago by someone who’s since left can suddenly block client emails. Review rules regularly and log changes so you know who did what and why.

4. Reputation issues and blacklists

If your domain has been used to send spam (sometimes via compromised accounts), receiving servers may blacklist you. That kills delivery until cleaned up. Remediation involves securing accounts, cleaning queues, and asking providers to delist you.

5. Third-party gateways and connectors

Many businesses route mail through third-party security or marketing platforms. A misconfigured connector or a change at the provider end can interrupt delivery. Treat those providers like part of your IT estate: ask them to prove mail flow, and require change notifications.

A practical checklist to troubleshoot fast

Use this checklist in order — it’s designed for a small IT team or a business manager delegating tasks. Each step focuses on restoring business continuity quickly rather than deep technical forensic work.

Step 1: Confirm the scope

Are some users affected or everyone? Is inbound or outbound mail failing? Narrowing this tells you whether it’s a tenant-wide issue, a user problem, or an external blockage.

Step 2: Check the Microsoft 365 admin centre

Look for service health notifications and message trace. Message trace tells you where a mail stopped and why — bounces, spam filtering, or successful delivery. If you don’t have access, ask whoever manages your tenant to run a trace for a recent failed message.

Step 3: Validate DNS records

Use a DNS checker (your IT person will have tools) to verify SPF, DKIM and DMARC are present and coherent. If you’ve recently changed hosting or DNS providers, records may not have followed across.

Step 4: Inspect mailbox and licence settings

Confirm affected users are licensed and that mailboxes aren’t full. Check forwarding rules and auto-replies — sometimes these are the culprits.

Step 5: Review transport rules and spam policies

Look for overly restrictive rules, recently added exceptions or quarantine settings. Roll back recent policy changes if you can, and test with a few addresses before wide change.

Step 6: Secure accounts

Compromised accounts often send large volumes of spam, harming your domain reputation. Force a password reset, enable multi-factor authentication, and review recent sign-in activity.

Step 7: Communicate to customers and staff

While you sort it, tell customers what’s happening and offer alternatives: a phone number or a temporary contact address. Keeping people informed preserves trust and reduces chasing.

If you’d rather not handle this in-house, consider a managed approach — many UK businesses benefit from external Microsoft 365 support that focuses on continuity and compliance rather than ticket-based firefighting. For example, a simple search for local Microsoft 365 support often shows options that combine proactive monitoring with fast remediation; you can find a relevant service here: Microsoft 365 support for business.

When it’s time to call in outside help

If the issue affects customer-facing emails, legal notifications or payroll, get external help immediately. Also call in help if you’ve got repeated blacklisting, suspicious account activity, or an incident that touches personal data — GDPR demands rapid response and clear audit trails.

Good external support offers more than a patch: they stabilise mail flow, document changes, and hand over a checklist so your in-house team can operate with confidence. That saves time and money in the medium term and stops the same problem from recurring.

Preventative measures that save grief later

Once delivery is fixed, harden your setup to reduce future incidents. Regularly review DNS records after domain or hosting changes, enforce multi-factor authentication, set up automated monitoring and alerts for failed delivery spikes, and schedule annual or bi-annual tenancy audits. For UK businesses, add a review of any supplier connectors and ensure your policies meet local compliance expectations.

These steps don’t require you to become an admin guru — they require disciplined governance. The payoff is calmer mornings and fewer “where’s that invoice?” emails from the sales team.

FAQ

How long does it take to fix common delivery problems?

Simple issues (licence problems, full mailboxes, misapplied rules) can be fixed in an hour or two. DNS-related changes may take up to 48 hours to propagate, though in many cases you’ll see improvements sooner. Complex reputation clean-ups or delisting from blacklists can take several days.

Will email disappear if Microsoft 365 has an outage?

Microsoft maintains robust redundancy; total outages are rare. Most delivery problems are configuration or account related. That said, have fallback communication channels and documented procedures so your business can carry on if something goes wrong.

Can I prevent my domain from being blacklisted?

You can reduce risk by securing accounts, limiting who can send bulk mail, monitoring outbound volumes and honouring unsubscribe requests. Quick detection and response to suspicious activity are the biggest preventive measures.

Is GDPR a concern when fixing email issues?

Yes. If delivery problems involve personal data or potential breaches (for instance, if accounts were compromised), follow your data breach procedures. Record investigation steps, who had access, and remediation measures — this audit trail is important if you need to report to the ICO.

Final thoughts

Fixing Microsoft 365 email delivery issues is rarely glamorous, but it’s vital. Prioritise restoring business flow, secure accounts, and put simple checks in place to stop the problem recurring. The goal is fewer interruptions, faster payments, and a calmer inbox for everyone.

If you prefer to outsource the heavy lifting and focus on running the business, look for support that delivers measurable outcomes: less downtime, lower risk and fewer staff hours spent firefighting. That’s the kind of calm that saves time and money — and gives you a bit more credibility with customers and suppliers to boot.