Troubleshooting mail client issues

Have issues connecting Fastmail with your third-party mail client or app? Below, we have troubleshooting tips for the most common problems our users encounter with third-party apps.

Basic connection issues

If you are unable to connect to Fastmail using your third-party app, there are a few things you can check to work out where the problem is occurring.

Check our operational status

If Fastmail is experiencing an issue at our end, we will post to our status dashboard with details and updates as we have them. If there is no information on this page, the problem may exist elsewhere.

Check if your account is overdue

If your account's subscription is overdue, sending and/or receiving mail could be disabled for your account. Third-party apps will not be able to connect to your account in this case. Please go to your Settings → Billing & Plan screen to check that your subscription is up-to-date.

Check for password errors

To access your Fastmail data using a third-party app, you must set up an app password for the app/program on your device.

Note: Basic plans do not support third-party app access, so you will not be able to create and use an app password if you are on a Basic plan. If you are on a Basic plan, you can use Fastmail on a web browser or download the Fastmail app.

Make sure that the app password you created has the right level of data access: If you wish to send/receive mail, the app password should have access set to Mail, Contacts & Calendars or Mail. You can check this by going to Settings → Privacy & Security → Integrations screen, finding the app password, and looking at what's in parentheses next to the app password's title.

If the app password doesn’t have the necessary access, you can create another app password with access set to Mail, Contacts & Calendars or Mail (IMAP/POP/SMTP), then replace the password in the email app with the new one. You can delete the old app password, as it is no longer needed.

Check your server settings

The standard server name and port details for setting up access to email, contacts, calendar, and files for your Fastmail account are provided in our Server names and ports help page. Check your app's server settings, and make sure that what you've entered follows the instructions on that help page.

Contact our support team with additional details and/or a traceroute

If you have gone through all the steps above and are still not able to connect your Fastmail account to your third-party app, contact our support team for further assistance.

It will greatly speed up the resolution of your issue if you let our support team know what you have already tried to fix the problem.

If you’re seeing email syncing/sending issues with your email app, it would also be very helpful if you were able to include a connection test of the connection between your computer and Fastmail. Below, we have instructions for running a connection test in Windows and Mac:


  1. Click the Start button and begin typing command prompt. When Command Prompt appears as a program, click it to open it.
  2. Copy and run the following commands on Command Prompt:
    1. telnet 993
    2. telnet 465
  3. Copy and paste the output along with your description of the problem and send it to our support team.

If you see the message 'telnet' is not recognized as an internal or external command, you will need to follow the step below to enable telnet on your Windows machine:

  1. Click the Start button and search for "Turn Windows features On or Off."
  2. Enable the checkbox for Telnet client.
  3. Click OK.

Once you've enabled telnet, please run the first set of commands again.

Mac or Linux

  1. Open the Terminal application. To find this application on Mac, open Launchpad and begin typing terminal.
  2. Copy and run the following commands on your Mac Terminal:
    1. nc -v 465
    2. nc -v 993
  3. Copy and paste the output along with your description of the problem and send it to our support team.

Email app certificate errors

If your email software reports something like "Unknown CA Certificate," this means that you don't have the latest root CA certificates installed on your computer or in your email/browser app. You may be able to fix this by downloading the latest updates to your operating system. Alternatively, you can install the missing root certificate into your app directly. Whether or not this is possible will depend on your app, so we recommend reading your app's documentation to find out.

The certificate can be downloaded from DigiCert's root certificate page. The required certificate is called "DigiCert Global Root CA."

If you’re still facing issues, please contact our support team with the screenshot of the error message you’re seeing.

Folder names appear differently in the app

The folder list on your Fastmail account may display differently on your local mail app to how it appears on the web, especially if special characters such as . or ^ are used.

This is because your third-party email app is accessing the raw IMAP data directly from the server. Due to some technical reasons, we store data in a particular way on the server, and then display it in a way that makes sense to the user. Some email apps do not do this conversion and instead display the raw data from the server.

These raw folder names also need to be used in Sieve scripts, as Sieve does not perform the folder name translations.

Effect on dot (.) characters in folder names

If you create a folder called in the Fastmail web interface or our mobile app, you will see a folder of that name show up in the web interface.

However, it will show up as i^really^like^dots in some IMAP apps. (This is the format you'd need to use in Sieve.)

Effect of caret (^) characters in folder names

The Fastmail web interface prevents you from creating a folder name with a ^ in it, as this is a special reserved character.

If you create a folder with carets in it via an app/client, Fastmail will be unable to display it in the web interface. (Many email programs will also refuse to save it.)

Unable to see shared folders

Some mail apps can't handle the special characters Fastmail uses to indicate shared folders.

While they will always be displayed in the web interface and our mobile app, not all IMAP apps will display them correctly.

Contacts aren't synced on setup

When you set up your Fastmail account on your device using CardDAV for contact syncing, this does not sync any contacts already on your device back to Fastmail.

As an example, let's say you already have contacts in your iCloud account, and you add your Fastmail contacts to your iPhone. In this case, your Contacts app will show contacts in both, but it will not copy the contacts automatically between them.

Moving pre-existing contacts from your device to Fastmail

If you want to move contacts from your device into your Fastmail account, you can export the contacts from your device, then import the contacts into Fastmail using our import tool. You'll need to manually check for duplicates.

Your device will end up with two copies of these contacts: one from the CardDAV sync from Fastmail, and the original copy on your device. You can now safely remove the original copy and just use the synced contacts.

Why don't my fields match up when syncing with CardDAV?

When using CardDAV to sync your contacts from Fastmail to another mail client or app, you may notice that some fields don't match up, or that they are missing altogether. We have further information about this on our Troubleshooting CardDAV fields help page.

Known email app issues

Many email apps are known to be buggy, which is one reason why we generally recommend using our UI over third party apps. Here are some known issues with some of the popular email apps:

Microsoft Outlook takes several hours to sync

We sometimes see our users report that Outlook takes a long time to sync emails or gets stuck displaying the “Synchronizing Subscribed Folders” status. While we don’t know for sure what causes this, we suspect it is due to a caching issue with the Outlook app or a problem with the Outlook data file.

In most cases, reconfiguring your Fastmail account on Outlook from scratch fixes the issue. We recommend creating a data backup before reconfiguring Outlook. To back up the message folders locally, you can export the items to a .pst file that you can restore later if needed. You can create a data backup by following the instructions here.

Microsoft Outlook shows “Something went wrong...”

We have seen that, in some instances, Outlook fails to connect to our servers and returns a generic "something went wrong" error. Unfortunately, we don't know what causes this, and there isn't much that Fastmail can do about this from our end.

We have seen that the same setup for the same account will work from a different device. We have also seen some users report that the setup starts working after a while on its own, without any specific action from their end. 

Slowness due to too many emails in a single folder

Some IMAP apps may have slower performance if there are a lot of emails in a single folder, especially in the Inbox. For example, we have seen cases where email apps take a lot of time to sync when more than 50,000 emails are present in the Inbox. If you’re seeing slowness in your email app and have many emails in a single folder, please split the emails in that folder into different folders.

It is recommended to keep as lesser emails in your Inbox as possible, as that's the folder that you often access. If you have a lot of emails in your Inbox, you can store them in your Archive folder instead.

Thunderbird 115 OAuth2 CalDAV sync error

Some of our users have reported receiving errors when trying to sync their Fastmail calendar to Thunderbird 115 via OAuth 2. Unfortunately, we don't know what causes this. We are in contact with Mozilla, and we're working with them to resolve the issue.

We have seen some of our users continue to access their Fastmail calendars via Thunderbird. This can be done by uninstalling Thunderbird 115 and then adding your calendar manually using CalDAV on Thunderbird 102. 

Known issues with Apple apps and clients

We have a detailed Known issues with Apple apps and clients help page that lists issues we're aware of on Apple devices.

We also list some known issues with iOS notes on our Notes help page and our Troubleshooting notes help page.

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