And I suppose you would also object to someone referring to a missive hand-written in ink on a piece of paper, placed in an envelope and sent through a postal service as a brick and mortar letter.

No, I wouldn't. It's somewhat redundant, as I would never think that someone who said, "I sent you a letter" was talking about an email, but to me, "brick and mortar" has gained the metaphorical definition of "real world, as opposed to the Internet." Saying "brick and mortar letter" is extending the metaphor, not changing its meaning. You may now consider it an officially approved Phlatlander Phrase.