How to resolve outbound email failures due to Spamhaus blocksSummary<!-- /*NS Branding Styles*/ --> .ns-kb-css-body-editor-container { p { font-size: 12pt; font-family: Lato; color: #000000; } span { font-size: 12pt; font-family: Lato; color: #000000; } h2 { font-size: 24pt; font-family: Lato; color: black; } h3 { font-size: 18pt; font-family: Lato; color: black; } h4 { font-size: 14pt; font-family: Lato; color: black; } a { font-size: 12pt; font-family: Lato; color: #00718F; } a:hover { font-size: 12pt; color: #024F69; } a:target { font-size: 12pt; color: #032D42; } a:visited { font-size: 12pt; color: #00718f; } ul { font-size: 12pt; font-family: Lato; } li { font-size: 12pt; font-family: Lato; } img { display: ; max-width: ; width: ; height: ; } } When you configure a custom SMTP Server, your sender instance IP address may be blocked by Spamhaus or similar organizations. This article explains how to identify and resolve email delivery issues caused by IP blocks. To troubleshoot this issue: Retrieve a received email, export it, and inspect the header to collect all possible details. If all emails are rejected, this may not be possible.Check the email header with the Message Header Analyzer.Verify the reported IP with nslookup (available on Mac).Your mail admin may need to add the specific IPs to the allow list in your mail server.If your instance sends emails to other domains, they might also be affected but not reported. Check for bounced or missing emails in these cases.Run a test connection to verify that your instance can connect to the SMTP server. If the connection fails, work with your mail administrator to create an exception and allow the IP to connect for sending emails. Note: Requesting removal from Spamhaus typically takes about 24 hours. Related Links<!-- /*NS Branding Styles*/ --> .ns-kb-css-body-editor-container { p { font-size: 12pt; font-family: Lato; color: #000000; } span { font-size: 12pt; font-family: Lato; color: #000000; } h2 { font-size: 24pt; font-family: Lato; color: black; } h3 { font-size: 18pt; font-family: Lato; color: black; } h4 { font-size: 14pt; font-family: Lato; color: black; } a { font-size: 12pt; font-family: Lato; color: #00718F; } a:hover { font-size: 12pt; color: #024F69; } a:target { font-size: 12pt; color: #032D42; } a:visited { font-size: 12pt; color: #00718f; } ul { font-size: 12pt; font-family: Lato; } li { font-size: 12pt; font-family: Lato; } img { display: ; max-width: ; width: ; height: ; } } Spamhaus Slow outbound email processing when own SMTP server is used Custom SMTP Server fails sending emails with error "Local address contains control or whitespace" Emails sent from custom SMTP email account ProofPoint get stuck in a send-retry loop Enabling email delivery using Sender Policy Framework (SPF)