Emails remaining in outbox (some sending some not) SMTP error

I am having an intermitent issue with my gmail account. I receive the following error but only occasionally and emails remain in the outbox and do not send. The error generally occurs 3-4 times per day, so emails normally send between these errors, but the last 24 hrs no  emails are sending, but I can receive.

xxxx@gmail.com: Connecting to xxxx@gmail.com failed
[SMTP] An attempt to connect to xxxx@gmail.com failed. This could be caused by temporary server unavailability or incorrect settings. Do you want to check the settings?

These are the settings. Please also note I am using an Alias for gmail.

Please help out as quickly as possible.

Thanks

Well, the server settings are as they should be. 

Does this happen with messages sent using the main address, or is it just those sent on the alias?

Thanks Gary. I have since disabled the avast email protection and it now works as expected.