MassEmail#sendQueueItem: [500] Invalid header value detected [] []

[wtconseil]

hi Yuri,
the mass mail campaign is "completed" and most the mails have been sent.
I don't know on which specific mailbox i got this Espo.ERROR log message

but i get it a few time

[yuri]

Hi,

Are emails sent out?

[wtconseil]

Hi there

during a newletter campaign, i can see in the log

[2018-01-10 00:00:04] Espo.ERROR: MassEmail#sendQueueItem: [500] Invalid header value detected [] [] 

any idea about this issue ?
thanks