Outbound emails are failing with invalid addresses blocked by Spamhaus with custom configured SMTP ServerSummaryWhen configuring a custom SMTP Server, the sender instance IP address may get blocked by https://www.spamhaus.org/ or similar organisations. Requesting removal from spamhaus takes about 24 hours. The instance administrator will need to 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 to get.The email header can be checked with https://mha.azurewebsites.net/, and the reported IP can be checked with nslookup (available on mac). Most likely the instance mail admin will need to allow-list the specific IPs in their mail server. If an instance sends out emails to other domains, they might also be affected but not reported. In these cases check if the instance has any bounced emails, or missing emails.Related LinksSlow 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