Troubleshoot slow outbound email when using custom SMTP servers

[John_Yates]

You’ve configured your instance to use your own SMTP server instead of the default ServiceNow mail servers. Now, outbound email feels sluggish, and you may notice frequent SMTP sender alerts or a growing queue of email [sys_email] records waiting to be sent.  

This article helps you identify where the delay is happening for non-group emails, understand why a custom SMTP server can be slower than the default ServiceNow mail servers, and gather the right data so your email team (and ServiceNow Support, if needed) can help you resolve it.  

Who this is for 

• ServiceNow admins 
• Platform owners 
• Email or messaging admins who manage SMTP servers, gateways, or Office 365 or Exchange 
• Anyone troubleshooting delayed notifications from ServiceNow 

A likely cause of outbound email slowness 

Email delays can have multiple causes, but often the custom server can't handle the volume of emails from your instance. For example, if your instance generates 100 emails per minute during peak hours, your email server must be able to accept 100 emails per minute. If the server capacity is inadequate, you may notice symptoms like:  

• Notifications arrive minutes or hours after an action was taken.
• Workflows or flows appear to run on time, but emails arrive much later.
• Email records show as sent or processed but the mailbox receives them much later.
• Email diagnostics show spikes in the outbound queue or longer processing times .

 

How outbound email works with a custom SMTP server 

When you use the default ServiceNow mail servers, your instance delivers email directly to an infrastructure that’s optimized and located physically near the instance host. 

When you switch to custom SMTP server, the flow looks more like this: 

1. ServiceNow generates the email and delivers it to your configured SMTP endpoint. 
2. Your SMTP server (and any downstream gateway, filter, or cloud service) accepts the message. 
3. Your mail system applies rules you’ve configured like: 
   • Spam checks 
   • Malware scanning 
   • Data loss prevention (DLP) or content inspection policies 
   • Throttling and rate limits 
4. The message is then relayed to the recipient’s mail server. 

Any slowness after step 1 is generally outside of ServiceNow and occurs in your mail infrastructure. 

To get a clearer understanding of the root cause, as well as gather the needed data for your email administrator, let’s go through basic troubleshooting steps.  

Confirm you’re using a custom SMTP server

Let's check that the SMTP server settings are configured correctly. 

1. Go to System Mailboxes > Administration > Email Accounts. 
2. Look for the SMTP account marked Active. 
3. Validate: 
   • Type is SMTP (not POP/IMAP). 
   • The server name, port, and security or authentication settings match your mail team’s documented values. 
   • The From address and display name are what you expect. 

Check your email diagnostic dashboard  

Depending on your instance version and plugins, you may have an Email Diagnostics or similar dashboard. Use it to get a broader view: 

• Is email sending reported as operational? 
• Are average processing times trending up? 
• Is the outbound queue length growing over time? 

If diagnostics show healthy processing times and no errors, that’s another hint the slow down is happening after ServiceNow hands the email to your SMTP server. 

Check how ServiceNow is processing outbound email 

Let’s take a closer look at the base system to see how emails are being processed and how quickly

1. Go to System Logs > Emails (sys_email table). 
2. Filter on [Type] [=] [Outgoing] and look at the recent records. 
3. Pay attention to: 
   • State (send-ready, sent, failed, and so on.) 
   • Created time versus Delivered time (or when the state changes) 
   • Any error or response messages 

Evaluate the following: 

Symptom	Cause
Emails sit in send-ready state too long	Delay is likely due to processing, connection, or retries
Emails arrive late in the recipient mailbox	Due to SMTP server or gateway

Collaborate with your email server administrator 

This is where your email, security, or network teams are essential. The more concrete data you bring them, the faster they can isolate the issue. 

Share: 

• A few sample emails containing Message-ID, sys_ids, and recipient addresses  
• A timeline of when the delays started 
• Approximate time delays between the workflow action time and the email arrival time for those samples 

Ask them to check for: 

• Queues and delays on your SMTP server or mail gateway 
• Message logs received from the ServiceNow IP or hostname 
• Rate limits or message caps applied to your ServiceNow sender account 
• Whether messages are unexpectedly routed through extra filters or other servers (as in "server hops") 

Wrap up 

Using your own SMTP server gives you more control, but it also means your email performance depends on your mail environment. 

By checking the outbound queue and reviewing email diagnostics, you can focus on where the delays happen to resolve them more quickly.  

 

Additional Resources   

• Troubleshoot slow outbound email processing with custom SMTP servers 
• How to use Preview Notification to troubleshoot undelivered emails 
• Verify a user's email notification preferences 
• Email and SMS notification  
• Email diagnostics dashboard