Configuring SMTPS in Virtualmin

Under System Settings > Server Templates (and under the dropdown menu “Edit template section”) > Mail client auto-configuration. In the section Thunderbird auto-configuration XML, under the heading outgoingServer, I changed the port from $SMTP_PORT to $SMTPS_Port and selected XMLtemplate below as opposed to Automatically generated. My XML template is therefore as follows:

<?xml version="1.0" encoding="UTF-8"?>

<clientConfig version="1.1">
<emailProvider id="$SMTP_DOMAIN">
<domain>$SMTP_DOMAIN</domain>
<displayName>$OWNER Email</displayName>
<displayShortName>$OWNER</displayShortName>
<incomingServer type="imap">
<hostname>$IMAP_HOST</hostname>
<port>$IMAP_PORT</port>
<socketType>$IMAP_TYPE</socketType>
<authentication>$IMAP_ENC</authentication>
<username>$SMTP_LOGIN</username>
</incomingServer>
<outgoingServer type="smtp">
<hostname>$SMTP_HOST</hostname>
<port>$SMTPS_PORT</port>
<socketType>$SMTP_TYPE</socketType>
<authentication>$SMTP_ENC</authentication>
<username>$SMTP_LOGIN</username>
</outgoingServer>
</emailProvider>
</clientConfig>

If the variable $SMTPS_Port doesn’t work for you, you can try manually entering port 465.

Src: https://serverfault.com/questions/605090/configuring-smtps-in-virtualmin