Hi @Rob , Welcome to our forum!
Does this issue occur recently? If so, have you done any specific operations before that?
And i notice that the issue is related to some emails, do they have something in common?
In addition, what's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook and Windows to the latest version. If both of them are the latest version, it's suggested that you could try to use the Microsoft Support and Recovery Assistant, which works by running tests to figure out what's wrong and offers the best solution for the identified problem.
We can also do the following troubleshooting:
- Considering the impact of some add ins, It is suggested that you try to start outlook in safe mode(win+R>type: outlook /safe) to check whether it works normally.
- If this is still the issue in safe mode, it is suggested that you could try to create a new profile(Control Panel>Mail>Show Profiles>add) and add the account for testing to check if there are some differences.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.