Unable to send emails in Outlook after upgrading to Windows 10

Upgraded the PC to Windows 10 and Outlook receives perfectly but every send sits in the Outbox forever or errors out. Nothing about the account changed, only the operating system underneath.

Same settings that worked last week. What does an OS upgrade break in the send path specifically?

Sending is the fragile direction after an upgrade because it crosses more machinery: the security suite's mail filter, the network stack and Outlook's own send receive configuration all sit between you and port 587, and an OS upgrade reshuffles all three. In order of hit rate:

1

The security suite first: upgrades reinstall or reset antivirus components and their outbound mail scanning proxies break in the process while inbound survives, producing exactly your asymmetry. Disable the AV's email or Outlook integration module temporarily and send a test. If it flows, update or reinstall the suite properly rather than leaving the module off.

2

Outlook's send receive settings next: the upgrade can corrupt the SRS file holding them. Close Outlook, rename the .srs in %appdata%\Microsoft\Outlook and restart, Outlook rebuilds defaults and stuck send behaviour frequently clears with it.

3

The network stack third: reset Winsock, which upgrades leave in odd states, from an elevated Command Prompt with netsh winsock reset, then reboot and test.

4

Still stuck, clear the Outbox of any half sent message blocking the queue, then verify the outgoing settings against the provider's current page, authentication ticked and port 587 or 465 with encryption, since some upgrades restore aged settings from backup. A fresh mail profile from Control Panel, Mail is the final broom if the account checks out.

Diagnostic shortcut for the impatient: send one message from the provider's webmail. Webmail working confirms the account and narrows everything to this machine's send path, which the four steps above cover in descending order of likelihood. Post upgrade send failures land on step one or two in the great majority of threads like this one.

The antivirus outbound proxy, first guess, first fix. Its upgrade compatibility update had been sitting in its own updater unnoticed. Installed, module re-enabled, Outbox flushed.