The IRS has posted on their MeF status page that they are having "ETIN Retrieval Errors" and "Read Timed Out Errors" in MeF.
What this means is that transmitters are having trouble transmitting and picking up acks.
ACK turn-around time has gone from minutes to almost an hour.
IRS is working the issue as a high priority. ACKs may be delayed.
--- Phil Drake