Erroneous permanent email errors

I periodically get returned email with permanent errors.  These can be from a distribution list or to individuals.  The addresses on the distribution are from comcast, aol, gmail, snet, and hotmail.  The individual failure is most often from aol.  Most of the time, all the addresses work.  Once in a while, 5 or 6 kick back, not always the same ones.  However, I do not believe either the comcast nor hotmail addresses ever failed.  This is my personal email from home and is going to other personal email systems.  The errors occur when I am the originator as well as when I am replying to all.  When an error code is included in the delivery failure notice, it is usually 554 5.7.9, once in a while it is 550 5.7.26.  The text of the message always refers to a permanent error.  But the next time I send, it goes through so it is not permanent.  This has been going on since at least April when I started recording it.  

Please help!