Your SMTP provider may have daily limits on how many emails can be sent. Gmail, for example, has a limit of approximately 100-150 emails per day when connected to the server from a remote email client.
A List of SMTP and IMAP Server
Important Update In Regard To Using Gmail, Yahoo, or Outlook As Your SMTP Provider:
In regard to setting up Amazon SES:
- Use the correct server name listed on the SMTP Settings page
- Use port 465
- Use the IAM Username & Password you created ( and hopefully saved because you can't view it again )
- Use the Amazon AWS Root User Email Address
Managing Your Amazon SES Sending Quotas
Moving Out of the Amazon SES Sandbox
In regard to setting up Yahoo:
Yahoo has some security checkpoints because of that we aren't being able to use yahoo SMTP easily as other providers. We won't be able to do reply tracking with yahoo since yahoo is not allowing us to do that. We are going to take Yahoo down soon.
In regard to setting up Outlook / Microsoft Office 365:
It is common to get the error Authentication unsuccessful even though the two-step verification is OFF. Microsoft changed its security. Now a third-party app requires SMTP Authentication to be enabled. Here is the guide to enabling SMTP > https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365
If the above article is not helpful, please reach out to Outlook support to get the account authenticated.