Fix dead link to SMTP.md (#55)

SMTP.md does not exist. Update link to target Smtp.md to fix 404 error.

docs/admin/Mail.md

@@ -17,7 +17,7 @@ Several services work together. The table and diagram provide a summary of these
 
 | Service                         | Role                                                        |
 | ------------------------------- | ----------------------------------------------------------- |
-| [Postfix](SMTP.md)              | Receives and sends mail                                     |
+| [Postfix](Smtp.md)              | Receives and sends mail                                     |
 | [Dovecot](Dovecot.md)           | IMAP/POP3                                                   |
 | [rspamd](rspamd.md)             | Spam filtering, outbound milter (policy, DKIM)              |
 | [SpamAssassin](SpamAssassin.md) | Spam filtering, *does not provide outbound milter services* |
@@ -279,4 +279,4 @@ postfix reload
 
 More advanced [round-robin configurations](http://postfix.1071664.n5.nabble.com/round-robin-map-with-ipv4-and-ipv6-td104719.html) are available as well as sender-dependent transport maps that use sender domains to [bind to IPs](Smtp.md#splitting-delivery-ips) on-the-fly.
 
-Always keep **at least one IP address** on standby.
+Always keep **at least one IP address** on standby.