Your emails in your Outbox display a state of Couldn't send. When you hover over that label, the full error of Send failure - please ensure your credentials are up to date in mailbox settings appears.
- Email / Mailbox
Resolve this issue in your mailbox in User Settings.
- Hover over your initials at the bottom of the left-hand navigation and click Personal Settings.
- Click See all mailboxes.
- Find and click the affected mailbox.
- Click Disconnect.
- Re-connect your mailbox as if setting it up for the first time.
- Go back to your Outbox and retry the mailings that failed to send.
If the user has changed their password on their email account recently, that will cause Outreach to lose connection with their mailbox.