Guide · Microsoft 365
Sync stuck, search broken, mailbox full: which cause belongs to which symptom – and what you should never delete first.
Diagnosis
Outlook shows very different problems with very similar symptoms. Matching them up saves most of the search.
Usually a connection problem or a stuck synchronisation. First check whether new messages are visible in the web interface in your browser – if they are, the problem is the computer, not the server.
The search index is damaged or incomplete. A common annoyance after moves and major updates – it has nothing to do with the mailbox itself.
Usually a very large data file or an add-in. Add-ins from business applications, archiving solutions or phone systems are the usual suspects here.
A cached credential no longer matches, or a security policy is taking effect. Entering it again and again does not help – the old sign-in has to be removed.
Above a certain size, sending is blocked, while receiving may partly still work. Those affected often only notice when someone complains about an unanswered email.
Permissions were changed or were never fully granted. Access to a mailbox and the right to send on its behalf are two different things.
Important
Three pieces of advice that often appear at the top of online search results and regularly cost data.
From harmless to invasive. After each step, check whether the problem has been solved.
Frequently asked questions
Technically, mailboxes of 50 GB and more are common today. In practice, Outlook becomes noticeably sluggish from about 25 to 30 GB, especially when searching and during the first sync on a new computer.
The real problem is rarely a large number of messages; it is large attachments. A mailbox where quotes and photos have been piling up for years grows quickly. It makes more sense to move attachments to your file storage and keep only the correspondence in the mailbox.
We advise against it. PST files usually sit on a single computer, so they are not included in the server backup, and they are considered fragile – especially when stored on a network drive, which Microsoft explicitly does not support.
The better route is archiving on the server or an archiving solution that stores data in a tamper-proof, audit-compliant way. Existing PST files should be imported and then retired.
The search index is built locally and does not move with you when you change computers. After the move it starts from scratch – with a large mailbox this takes hours to days, and in the meantime search returns incomplete results.
That is not a fault; it is normal. If it has not improved after a week, the index is damaged and needs to be rebuilt.
An OST file is a local copy of a server mailbox – it is synchronised and can be recreated at any time. A PST file is standalone: whatever is in it exists only there and is not synchronised anywhere.
This leads to the most important rule: an OST can be deleted if necessary; a PST never without a backup first.
For many workstations, yes. Local data files, profile problems and index issues disappear, and access works the same from every device.
Arguments against switching are business applications that hook into Outlook, extensive local archives and users who rely heavily on rules and folder structures. The best way to check is with two or three people in their daily work before deciding for everyone.
Dipl.-Ing. Mohamed Khater
Dipl.-Ing. Mohamed Khater founded INFONET Computer GmbH in Cologne in 1992. Outlook is by far the most frequent program in support calls – and the place where acting too hastily most often costs data.
Free assessment
If the same problems come back every month, the cause is rarely the individual workstation. We look at mailbox sizes, add-ins and permissions as a whole.
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