email received, but stuck in outbox - no error

I have em client set up, and on only one of my accounts, the emails receive okay. But they will not send out, they stay stuck in the outbox. No attachments on them. No error pops up. I have closed and restarted EM. I have deleted the email account, and reset up. I have other email accounts in EM, and they all work correctly.
See below - name marked out for security -
Operations log says:

eM Client 10.3.2619+1e54f4a61e (Windows)

7:40:07 AM ******@suntana.com IMAP Synchronizing folder ‘/Calendar’
7:40:07 AM *****@suntana.com IMAP Synchronizing folder ‘/Contacts’
7:40:07 AM ******@suntana.com IMAP Synchronizing folder ‘/Conversation History’
7:40:07 AM ******@suntana.com IMAP Synchronizing folder ‘/Drafts’
7:40:08 AM ******@suntana.com IMAP Synchronizing folder ‘/Email_Templates’
7:40:08 AM ******@suntana.com IMAP Synchronizing folder ‘/Journal’
7:40:08 AM ******@suntana.com IMAP Synchronizing folder ‘/Junk Email’
7:40:09 AM ******@suntana.com IMAP Synchronizing folder ‘/Notes’
7:40:09 AM ******@suntana.com IMAP Synchronizing folder ‘/Outbox’
7:40:09 AM ******@suntana.com IMAP Synchronizing folder ‘/Send_Later’
7:40:10 AM ******@suntana.com IMAP Synchronizing folder ‘/Sent’
7:40:11 AM ******@suntana.com IMAP Synchronizing folder ‘/Tasks’
7:40:12 AM ******@suntana.com IMAP Synchronizing folder ‘/Trash’
7:40:13 AM ******@suntana.com IMAP Synchronizing folder ‘/Inbox/saved’
7:40:19 AM ******@suntana.com [SMTP] MailClient.Accounts.AuthenticationAbortedException: Authentication aborted
at MailClient.Protocols.Smtp.SmtpSendCommand.Authenticate(WorkerStatus status)
at MailClient.Protocols.Smtp.SmtpSendCommand.Connect(WorkerStatus status)
at MailClient.Protocols.Smtp.SmtpSendCommand.Execute(WorkerStatus status)
at MailClient.Commands.Command.Process(WorkerStatus status)
RunInPlace = False
GUIStatus_Exception_Reported = True