On évite les décalages à l'aide de box-shadow

webmail-pages/webmail.css

@@ -29,14 +29,13 @@ main article p {
 width:80%;
 margin-left:10%;
 margin-bottom:2%;
-background:#FFFFFF;
-border:dashed ;
-border-color:#00419b;
-border-width:2px;}
+background:#FFFFFF; 
+box-shadow: inset 0 0 0 2px rgba(45,81,144);
+}
 
 main article p:hover {
-border-width:4px;
-border-color:#5aa120;}
+box-shadow: inset 0 0 0 6px rgba(45,81,144);
+}
 
 main article p a img {
 width:96%;