A. Mike DeBlasio, a masonry contractor in Littleton, Mass., responds: Efflorescence occurs when soluble salts in the masonry are dissolved by moisture being driven through the material via vapor transmission or hydrostatic pressure. When the solution reaches the surface and evaporates, a salt deposit — efflorescence — is left behind.
However, with efflorescence you'd see more widespread staining at grade and above. The problem you describe is more likely caused by contact between aluminum and the brick and mortar. Mortar is highly alkaline; it will react with an aluminum mailbox or window frame, breaking down the metal and causing staining.
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