New: add best structure of mail

dev/mail/dolibarr_mail_universal.txt

@@ -0,0 +1,49 @@
+Content-Type: multipart/mixed;
+ boundary="------------090104020208060008040809"
+
+This is a multi-part message in MIME format.
+--------------090104020208060008040809
+Content-Type: multipart/alternative;
+ boundary="------------030909020408050302070704"
+
+
+--------------030909020408050302070704
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: quoted-printable
+
+
+... plain text ...
+
+
+--------------030909020408050302070704
+Content-Type: multipart/related;
+ boundary="------------000403070904000609020709"
+
+
+--------------000403070904000609020709
+Content-Type: text/html; charset=UTF-8
+Content-Transfer-Encoding: quoted-printable
+
+... html text ...
+
+--------------000403070904000609020709
+
+... image inline ...
+
+--------------000403070904000609020709
+
+... image inline ...
+
+--------------000403070904000609020709-- (end multipart/related)
+
+--------------030909020408050302070704-- (end multipart/alternative)
+
+--------------090104020208060008040809
+
+... attach-file ...
+
+--------------090104020208060008040809
+
+... attach-file ...
+
+--------------090104020208060008040809-- (end multipart/mixed)