I am having problems in sending emails with 1MB and larger attachments. The emails can’t be sent and stays in outbox, then an error message “[SMTP] An attempt to connect to “my email” failed. This could be caused by temporary server unavailability or incorrect settings. Do you want to check the settings?” is generated. I have no problems in sending emails with less than 1MB attachment. I am using Windows 10 now and I have other PC which is using Windows 8 which does not have this problem.
Maybe try disabling your anti-virus application, then try again.
You could also check that you have the correct SMTP settings. Go to Menu > Tools > Accounts and click on the SMTP tab for your account. Make sure you have one of the following port/security policy combinations:
Port 587 = Force usage of SSL/TLS
Port 25 = Use SSL/TLS if available
Port 465 = Use SSL/TLS on special port (legacy)
The underlined one is the best option for security if your server supports it. SMTP port 465 was reassigned and should no longer be used for SMTP, though many mail providers still recommend and use it.
I tried all the security policy options but nothing worked.
Disabling the anti-virus application worked and solved the problem.
Thanks for the help!