When using Gmail to send email from Dynamics 365, emails are not sent and appear in a Pending Send status. If you open the mailbox record in Dynamics 365 and click Alerts, you see the following warning:
"An unexpected error occurred while sending the email message [Email Subject] through the mailbox [Mailbox Name]. The owner of the associated email server profile [Email Server Profile name] has been notified. The system will try to send the message again later.
Email Server Error Code: Smtp server returned GeneralOutgoingEmailServerError GeneralFailure exception."
This message may be logged if you exceed the number of emails that Gmail allows you to send from a single mailbox.
Refer to the following article for details about sending limits from Gmail:
If you need to send bulk emails, a solution such as Dynamics 365 for Marketing or another solution designed for sending large numbers of emails should be used.