Storing email inbox information
Joining the mailbox table to a "FolderNames" table using a foreign key will be problematic if any particular user wants to change the name of any given folder, without changing that name for everyone else.  You could get around this by having a nullable CustomFolderName column in the mailbox table.

Depending on the expected number of accounts, storing the name of the mailbox in the mailbox table is probably not a deal-breaker for performance, unless perhaps there are details you haven't shared with us.  When you say these are "smallish strings", don't specify the column as having a maximum length of 1000 characters if your app will limit the folder names to 15 characters.

Overthinking optimization early on can be burdensome to the development effort.  Focus on the database, just don't go entirely overboard at the very earliest stages of development.  You're quite likely to change the structure as development progresses anyway.  Ensuring you use the correct data types as appropriate is going to be far more important.  For instance, please don't store dates in string columns.  Use a date column.  If there is going to be a time element, use a date/time column.  Don't use 128 bit integers where 32 bit will suffice.