Some recipients (Bittrex, Kraken and others) may require transaction memo to be provided for correct forwarding of your lumens.

In cases when memo is required by receiving end, failure to include the memo with your transaction may result in your funds being lost forever and LOBSTR won't be able to help.

Entering a memo is usually not required when sending to a muxed address (starting with M). You can still attach a memo to this transaction if you’d like.