Uploading emails from Centurylink.net

Slow upload or synchronization of messages from a Centurylink.net server into Outlook is typically caused by either server-side performance issues, network problems, or Outlook performance/sync issues.

Use the following checks and mitigations:

1. Check Microsoft 365/Exchange Online service health (if Centurylink account is being delivered into a Microsoft 365 mailbox)
   • Sign in to the Microsoft 365 admin portal with admin credentials.
   • Open Service health and verify whether there are any ongoing Exchange Online issues affecting mail flow or performance. If there is a service incident, performance should improve after it is resolved.
2. Check local network and connectivity
   • Verify that the internet connection is stable and not heavily loaded (for example, large downloads, VPN, or streaming running in parallel).
   • Check for issues with local proxy, firewall, or ISP that might throttle or delay IMAP/POP/SMTP/Exchange connections.
3. Reduce Outlook load and improve client performance
   • Large mailboxes and many items in the Inbox can slow Outlook’s synchronization with the server. Move older messages to local folders to reduce the number of items that must sync:
     1. In Outlook for Mac: Outlook > Preferences > General.
     2. Under Sidebar, clear Hide On My Computer folders.
     3. In Mail view, drag older or less-used messages from the server Inbox to On My Computer folders.
   • Consider creating rules to automatically move or archive older messages to local folders so the online Inbox stays smaller.
4. Check for add-ins or security software that may slow sending/receiving
   • Web add-ins or security tools that inspect mail can delay sending and synchronization.
   • Temporarily disable non-essential Outlook add-ins and test performance.
   • If a third-party security product (for example, DLP or content inspection) is attached to Outlook or the mailbox, pause it and test. Some tools can create excessive mailbox properties and cause sync issues.
5. Verify that messages are not being delayed by rules
   • In Outlook, open Rules / Manage Rules & Alerts and check if any rules use “defer delivery by # minutes”.
   • If such a rule exists and is not required, edit the rule and clear the defer delivery by a number of minutes action so messages are not intentionally delayed.
6. Check for known sync issues (if using Exchange/Outlook for Microsoft 365)
   • If emails are stuck in the Outbox or sync errors appear (for example, error 80040611 in the Sync Issues folder), this can indicate mailbox property quota problems caused by third-party tools.
   • In that case, pause the problematic tool (for example, a DLP agent) and contact the service provider or Microsoft 365 support to perform a mailbox move and recreate the Outlook OST as described in the guidance.

If, after these steps, uploads from Centurylink.net are still extremely slow, collect details (protocol used: IMAP/POP/Exchange, approximate mailbox size, whether delays occur only on this network or all networks) and contact Centurylink support and/or Microsoft 365 support with that information for deeper server-side investigation.

References:

• Outlook performance is slow in the Microsoft 365 environment
• Outlook is running slow
• Outlook sync issues cause emails to be stuck in the Outbox and undeliverable mail items
• Email messages remain in the Outbox because of Deferred Delivery rule