Send mail jobs are getting frozen

Hello Experts,

Email notification jobs to notify the status of request in Personwantsorg is getting frozen with below error message. We logged onto server and can't find any issues. Please check and assist us ASAP:

Job Names: (most of all the send mail jobs)

AJG_Attestation_AttestationHelper send mail collection of pending tasks for approver
AJG_Attestation_AttestationHelper send mail new task for approver
AJG_Attestation_AttestationHelper send mail to remind approver
AJG_ESS_PersonWantsOrg send mail when step granted
AJG_PersonWantsOrg_Send_Mail_Approval_Notification

Error Messages:

ErrorMessages = (2019-07-10 07:00:38.303) [MailKit.ServiceNotConnectedException] The SmtpClient is not connected.
at StdioProcessor.StdioProcessor._Execute(Job job)
at VI.JobService.JobComponents.MailComponent.Activate(String task)
at VI.JobService.JobComponents.MailComponent._SendRichMail()
at VI.JobService.JobComponents.MailComponent._Send(MimeMessage message, MailSecurity security)
at MailKit.MailTransport.Send(MimeMessage message, CancellationToken cancellationToken, ITransferProgress progress)
at MailKit.Net.Smtp.SmtpClient.Send(FormatOptions options, MimeMessage message, CancellationToken cancellationToken, ITransferProgress progress)
at MailKit.Net.Smtp.SmtpClient.Send(FormatOptions options, MimeMessage message, MailboxAddress sender, IList`1 recipients, CancellationToken cancellationToken, ITransferProgress progress)

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