Can't send outgoing messages in Mac Mail

I have a Gmail account which I access using Mac Mail. I still am able to receive messages but cannot send messages in Mac Mail. I receive the following message for each unsuccessful attempt to send:

Cannot send message using the server smtp.gmail.com: (and my e-mail address) The SMTP connection to server "smtp.gmail.com" failed.

I called Google for assistance and was on hold for a very long time. I don't believe I've done anything wrong lately to cause this dilemma. Can anyone offer some assistance? It will be greatly appreciated.

As stated before, all this means is your outgoing server credentials are incorrect in the Mail App.

In order to check the outgoing server's credentials go to Mail menu > Preferences > Accounts > find your Gmail account. Click on your gmail account then under "outgoing server" check what server is chosen by default then click "edit smtp server list" at bottom.

Once there double check all the server info. Make sure it has your email spelled correct and password correct, make sure authentication is set to "password" then make sure you use SSL and use port 465 or 587.

I had the same problem. All my settings seemed correct. "Connection Doctor" indicated my username/password was incorrect but I thought it was weird since I did get incoming mails. Anyhow, after I re-typed my email address/pw inside the "edit smtp server list > Advanced", the outgoing mail went out successfully. BTW, I have 2 gmail accounts configured in Mac Mail, only one of them had this issue. Cheers~

This is a common issue with any IMAP account when setting up through Mac Mail. If you are able to receive messages BUT NOT SEND (GMail, Hotmail, Live, etc), the answer is simple :)

Click Mail > Preferences > Accounts (select yours) > (select) Account Information > Click the Drop Down (smtp.gmail.com, smtp.live.com, etc), select Edit Server List > Advanced > Click the Drop Down Box under AUTHENTICATION, select PASSWORD

To find this menu, be sure to uncheck, Automatically manage connection settings.

Enter your FULL email and password, and the checkbox for default ports and SSL should be checked.

Basically, you skipped the authentication step when setting up the outgoing emails, so you're sending an email that connects to a server with no credentials, hence the "unable to send" message. Hope this helps someone who is new to Mac OS!