[TYPO3] problems with direct mail - only a part of the recipient-emails are forwarded

[Timo Rehmann]

hello folks,

following problem occurs, while sending a newsletter with directmail:

the given recipient-list has around 15000 entries, but only 5800 emails 
are sent. this behaviour is reproducable.

database-entries, recipienlist and ext-config is checked (in fact we 
copied a correct working typo3-system to a new server), but phpmyadmin 
outputs a warning for the table fe_users:

"*Es sollte nicht mehr als ein Index des Typs INDEX für die Spalte `pid` 
gesetzt sein*"
translation:
"there should be not more than one index of type INDEX defined for the 
column pid"

this is the overview-list of the indices:

PRIMARY PRIMARY 208  uid
parent         INDEX      4      pid
username    INDEX      208  username
is_online     INDEX      12    is_online
pid             INDEX      208  pid/username

could it be, that this error/warning is responsible for the ridiculous 
behaviour of sending emails with direct mail?

if, can i delete one of the indices (which?)?

if not, what else could be the reason for the wrong behaviour of sending 
those emails?

the database itself was an 3.23.37 on the old server and is now 5.0.24a ...

thanks for any hint!

best regards,
timo rehmann