Cannot explain ''undisclosed recipients'' in sent email

It’s now two weeks that I’m trying eM Client, and so far I like it pretty well. I’m working with a IMAP account.
Nevertheless there is one annoying error that occurs when I’m sending a message to one specific contact, let’s call him Freddy.
Always when I send an email to Freddy, this email will show up in the sent folder, but it’s addressed to ‘‘undisclosed recipients’’. Now, when I check the send the sent folder in Thunderbird and in the Webinterface of my mail provider, it is correctly addressed to Freddy, only eM Client shows it as ‘‘undisclosed recipients’’. To make it more mysterious, this happens (so far) only with Freddy, every other contact is correctly displayed.
What could cause this?

Thanks
 

Hi Simon,
this is indeed a strange issue, could you please export one of these emails to ‘Freddy’ to an EML file and send it to me at rust@emclient.com so I can have a look at it?
Please attach a link to this forum topic in the body of the message so I can get back to you about it.

Regards,
Olivia