In Pegasus Mail:
Options - Sending Mail - Checkbox 'Suppress BCC listings when sending mail" should be checked!
To quote David Harris:
"Suppress BCC field listings in outgoing mail BCC (Blind Carbon Copy) is a useful, but poorly-standardized feature. There are at least four ways a BCC field could be written into a message:
It could be omitted altogether
It could be present, but contain no addresses at all
It could contain only each individual recipient's address
It could contain the addresses of all people receiving the BCC
All of these methods have adherents and detractors. By default, Pegasus Mail lists all the BCC recipients in the BCC field of mail it sends: if you would prefer that no addresses were shown in the field, then check this control. When this option is turned on, the BCC field will simply contain the text "(Suppressed)", without any addresses."