If the problem arises that MS Outlook is not able to send emails, then it can be solved very quickly. It will be as simple as changing any of the settings on your system.

Best ways to troubleshoot Outlook when you don’t be able to send mails

Source:- Webroot Support

Send the mail again
It may seem non-useful. But sometimes even sending the mail one more time solves the problem.

Firstly, do a right-click on the mail that has been stuck and point to Move.
Then, select the Drafts folder.
After that, check the mail in the Drafts folder.
Next, press Send.
Lastly, go through and note whether mail is present in the Sent list or in the outbox now.
Look on recipients mail address for twice
A small mistake in recipients address can stop Outlook from sending the mail. For instance, a mistake of a comma.

Look at your connection
Ensure that you are connected to the net. To do that, go through the given below instructions carefully.

Firstly, in the Outlook window see toward the corner side.
Observe that whether it is saying things like-Disconnected, Working Offline or Trying to connect.
Then, press the Send/Receive option.
After that, hit Work Offline.
Note that now you will be able to get ‘Connected to server’. It will be towards the end of the window.
Next, press the Send/Receive All Folder options. It will be under the Send and Receive group.
Lastly, go through to check whether mail is present in the Sent list or persisting in the outbox.
Check the attachments
In Outlook, there is a restriction on the size of the files that can get attached. For instance, it is 20 MB for the accounts over the net like for Gmail and 10 MB for business mail accounts.

Synchronize your password
Note that if you have modified mail password few days before over net, then you require even to do it in the Outlook too. Otherwise, you’ll not be able to send or receive mail.

Firstly, press File.
Then, tap on Account Settings.
Click on Account Settings from the drop-down menu.
Press on Email option.
After that, select your mail account.
Tap on Change.
Put the newly made password.
Tap Next.
Hit Close.
Press Finish.
Lastly, use F9 key on your keyboard for trying to send your mail.
Delete sent items

Firstly, go to the Send folder.
Then, remove all the mails.
Close the Outlook.
Then, restart it.
Check it by mailing one more time to check if the problem is persisting or not.

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