Salutation for a letter of recommendation to be uploaded to mathjobs

Most of my math recommendation letters do not have an opening salutation. They have the date at the top right, and the first sentence orients the reader with something like "I write to recommend Dr. X to you for..." If I recall correctly, this is fairly typical for such letters. I am more confident in saying it doesn't matter: in general, the format for these letters is all over the place, and no one cares.

There is the widely used

To whom it may concern:

and also

Dear Sir/Madam: