A (1) beside my username@fasmail.fm
 

I see a (1) by my username@fastmail.fm in Outlook 365. How do I clear it?

As far as I can tell, there isn't a Microsoft product called "Outlook 365". So I assume you have an Office 365 account and that you are using the web email account through the URL https://www.outlook.com. Is this correct? Or are you using the Outlook email client installed on your PC or mobile device?

Exactly where do you see your Fastmail email address with an adjacent (1)? A web login to Outlook normally accesses your outlook.com/live.com/hotmail.com email accounts. Do you have your Outlook account set up to access your Fastmail account? Or have you sent to or from your fastmail.fm account using Outlook?

In some cases Fastmail adds a number in parenthesis such as (1) (2) etc. to the subject of a message if you appear to have an email loop. This is when an email is sent from your Fastmail account in response to a previous email from your account which is looped back to your account. This can happen, for example, if you have a notification message which is delivered or forwarded back to your account, generating another notification message. The (1) (2) etc. addition to the subject prevents the loop from repeating past (5), I believe.

Bill

Office 365 on PC
In the Outlook client. It does not appear on the web client.
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Exactly where do you see the (1)? Is it on the subject line?

The account title "dlnewhouse@fastmail.fm (1)".

You can change the name of an account in Outlook by going to:
File>Account Settings>Manage Profiles>(yes)>Show Profiles>Outlook>Properties>Email Accounts

You then select (click) the email account you want to change, then click Change. Next click More Settings...and you can change the name of the account.

Microsoft often hides things in deep menus. I find this very aggravating.

Bill

Outlook has switched from pst files to ost files. For some reason the file for fastmail.fm is

C:\Users\Daniel Newhouse\AppData\Local\Microsoft\Outlook\dlnewhouse@fastmail.fm (1).ost

It doesn't seam to imply any technical difficulties.

The likely reason this occurred that that you set up your FastMail account multiple times, and the .ost file name Microsoft wanted to use was already used. The "(1)" was added to make the name unique.