smtpmail-send-queued-mail fails with error 504 5.5.2

[rameiko87@posteo.net]

Hello,

Could you please kindly help me with this? When I have 
smtpmail-queue-mail set to nil, sending emails works beautifully. I then 
change smtpmail-queue-mail to t: then the text of the email is saved in 
my queued email folder together with another file specifying the value 
of smtpmail-recipient-address-list (it contains the same address as in 
the To: header of my message). When I invoke smtpmail-send-queued-mail I 
get the following error:

Sending failed: 504 5.5.2 <usernameofmymachine@fedora>: Sender address 
rejected: need fully-qualified address

The strange thing is that usernameofmymachine@fedora would be the 
default address to appear in the From: header of C-x m, but
1. I set the variable message-user-fqdn (and it's different from 
"fedora"), so actually when I press C-x m I have 
usernameofmymachine@message-user-fqdn which is already different from 
usernameofmymachine@fedora
2. Before sending (that is to say, queuing), I do change the From: to my 
email address, and it appears correctly on the text file saved in the 
queued folder!

By the way, smtpmail-store-queue-variables is nil.

Please, help! Thanks so much.