Troubleshoot connection: Failed to login CONNECT_ERROR with "The remote certificate is invalid" error

Troubleshoot connection: Failed to login CONNECT_ERROR with "The remote certificate is invalid" error

Error dialog commonly shown

Mitigations

  1. Try disabling any and all antivirus locally on the machine. Make sure email scanning or monitoring is turned off specifically in antivirus software (it's usually a hidden setting). Even if the main antivirus is turned off, email scanning often is a setting that remains turned on.
  2. Try disabling firewalls or allowing the program to connect both from the local machine and on the network (network firewall).

What does this error mean?

What some antivirus or firewalls do is that they intercept the email TLS connection and by doing so present a certificate that's different from what's being presented by the email server the program is trying to connect to. In security terms, this could be considered a man-in-the-middle (MITM) threat.  

Why did this start happening all of a sudden?

We have seen increasing reports from late 2024 and early 2025 of this error with newer releases of Windows and/or Antivirus updates as they try to tackle email related Phishing or other types of threats posed by emails. While this is a good thing for the industry, the approach taken to "intercept" emails isn't ideal.

Future plans to mitigate this

We plan to release an update in version 3.5+ that will offer support for custom certificates and/or offering a prompt for the user to bypass this error.

Instructions to turn off email scanning with popular antivirus software

Microsoft Defender

Microsoft Defender

To turn off email monitoring in Microsoft Defender,  access your Windows Security settings, navigate to "Virus & threat protection," and then under "Microsoft Defender Antivirus options," you can adjust the "Scan emails" setting to "Off" to disable email scanning functionality

Key steps:
  1. Open the Windows Security app.
  2. Select "Virus & threat protection".
  3. Choose "Microsoft Defender Antivirus options".
  4. Locate the "Scan emails" setting and toggle it to "Off". 
Bitdefender

Bitdefender

To turn off email scanning in Bitdefender, navigate to the "Protection" section in your Bitdefender interface, then go to "Settings" within the "Online Threat Prevention" panel, and under "Email Protection," select the option to "Don't scan emails." 

Key steps:
  1. Open Bitdefender, 
  2. Go to "Protection, Select "Settings" within "Online Threat Prevention, and 
  3. Under "Email Protection," choose "Don't scan emails. 
McAfee

McAfee Security Center

To turn off email scanning for McAfee Security Center, you can do the following: 
  1. Open the McAfee Security Center
  2. Click Configure in the left pane
  3. Double-click E-mail and IM
  4. Click Advanced under E-mail protection
  5. Uncheck Scan outbound e-mail messages
  6. Click OK