If you receive, "We apologize, but your return processing has been delayed beyond the normal timeframe", does it usually mean I might receive a letter in the mail?

Yes, it is definitely possible you may receive a letter from the IRS, but it isn't a certainty.

 

The IRS is using new methods to detect tax related identity theft this year. You may get a letter asking you to verify your identity, add a 1095-A, or something similar.

 

The IRS won't contact you by phone or email, so be wary of phone calls claiming to be from the IRS. 

 

On the other hand, the IRS may already have the information they need to complete processing your tax return. if that is the case, the refund will be released when they are ready.