does setting ethernet speed wrongly affect mail

I notice that if our server ethernet card setting
is configured wrongly (say if the other end to the router is set
100Mb , full duplex whereas the card itself is not, like
100Mb, half duplex, autonegot. on) , the ftp file transfer speed
to/from other servers are much slower.

I wonder how this wrong setting would affect the mail transfer.
or it is not significant for the smtp

I ask since sometimes users reported that their mates has sent
them mail but they never receive them (and the checking of the
mail log files don't show any transaction at all) and there
are no bounced back messages to indicate the mail did n't go thru

Could this happen due to the wrong network card setting i
mentioned above or is just my imagination?