Some emails queued but not sending

Hi,

Some emails (maybe 15%) are queued but not sending from my Nop 4.1 site.
If I requeue one of those email it sends.

I checked the error log and I see the below:
Has anyone any ideas on what the issue is or how to fix?

Short message
Error sending e-mail. Failure sending mail.
Full message
System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.Internals.SocketExceptionFactory+ExtendedSocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond 92.52.113.135:25
   at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress)
   at System.Net.Sockets.Socket.Connect(EndPoint remoteEP)
   at System.Net.Sockets.TcpClient.Connect(IPEndPoint remoteEP)
   at System.Net.Sockets.TcpClient.Connect(String hostname, Int32 port)
--- End of stack trace from previous location where exception was thrown ---
   at System.Net.Sockets.TcpClient.Connect(String hostname, Int32 port)
   at System.Net.Mail.SmtpConnection.GetConnection(String host, Int32 port)
   at System.Net.Mail.SmtpTransport.GetConnection(String host, Int32 port)
   at System.Net.Mail.SmtpClient.GetConnection()
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   --- End of inner exception stack trace ---
   at System.Net.Mail.SmtpClient.Send(MailMessage message)
   at Nop.Services.Messages.EmailSender.SendEmail(EmailAccount emailAccount, String subject, String body, String fromAddress, String fromName, String toAddress, String toName, String replyTo, String replyToName, IEnumerable`1 bcc, IEnumerable`1 cc, String attachmentFilePath, String attachmentFileName, Int32 attachedDownloadId, IDictionary`2 headers) in C:\Users\Joe\Desktop\x\Libraries\Nop.Services\Messages\EmailSender.cs:line 153
   at Nop.Services.Messages.QueuedMessagesSendTask.Execute() in C:\Users\Joe\Desktop\x\Libraries\Nop.Services\Messages\QueuedMessagesSendTask.cs:line 69

I'd start with https://mxtoolbox.com/ and go from there

Issue solved.
In the email account configuration settings I made the below updates:
Host: mail.mydomainname.com (I updated this to my mail server name)
Port: 25 (I updated this to 587)
SSL: unchecked (I ticked the checkbox).
I saved and tested but was not working.
I re-entered the password and clicked 'Save password'
I saved and tested and it worked.

All emails now sending.