Try to get the configuration from the email provider
Try to contact http://autoconfig.emailaddressdomain/mail/config-v1.1.xml?emailaddress=emailaddress , e.g. http://email@example.com, as well as fallback http://example.com/.well-known/autoconfig/mail/config-v1.1.xml, and see whether that host/URL exists. The returned file must have the ConfigFileFormat as below.
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For faster and more reliable delivery, add firstname.lastname@example.org to your trusted senders list in your email software.