This tag filter selects questions which match the following wildcard tag pattern:
*mail* OR rfc* OR *smtp*
The filter is returning questions tagged php, which does not match any of the wildcard expressions.
Perhaps it is an unintended side effect of tag synonyms interacting with the filter algorithm. In this case someone has created a tag synonym of php named php-mail. It is correct for the expansion of the tag pattern to include php-mail. But it is wrong for the filter algorithm to return questions that are tagged with php, which does not match the wildcard expressions in the pattern.