try something like:
$mailtags; //variable holding your tags
$tags = split(',',$mailtags);
//if necessary, change the first parameter of split() from a comma
//to whatever character your tags are delimited by.
foreach($tags as $tag) {
if(eregi("@",$tag)) {
$tag = eregi_replace("<","<b><",$tag);
$tag = eregi_replace(">","></b>",$tag);
}