mulberry 4.0.8 - incoming emails don't show sender's domain anymore

hey there,

being now on mulberry 4.0.8 on mac os x el capitan 10.11.1 and before 
that on yosemite 10.10.5 for quite a while now emails of some senders 
aren't displayed with either the sender's respective email address or 
the receiver's respective email address anymore in mulberry but with 
@MISSING_DOMAIN - e.g. name@MISSING_DOMAIN instead of [email protected] 
either in "From:" of the sender or the "To:" of that specific sender.

the problem only exists in mulberry. with mail.app, thunderbird and our 
company's webmail the email addresses are displayed correctly.

currently it just happens with only a few senders, but it's a bit 
distressing.

i checked the senders's emails headers, and it appears that it's 
happening only with users that use thunderbird 38 either on a mac or 
windows; from two senders (of the same company) the domain is "missing" 
using these tb-versions:

User-Agent: Mozilla/5.0 (Windows NT 6.2; WOW64; rv:38.0) Gecko/20100101
 Thunderbird/38.2.0     meanwhile already User-Agent: Mozilla/5.0 
(Windows NT 6.2; WOW64; rv:38.0) Gecko/20100101
 Thunderbird/38.3.0

User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101
 Thunderbird/38.2.0


on the other hand another sender of that same company is displayed 
correctly using one of the tb-versions from above:

User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:38.0) Gecko/20100101
 Thunderbird/38.2.0


and here are the specs of one mac-user:

User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:38.0)
 Gecko/20100101 Thunderbird/38.2.0     meanwhile already User-Agent: 
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:38.0)
 Gecko/20100101 Thunderbird/38.3.0



well, does anybody have an idea what the problem might be, or should i 
just wait and hope that it will disappear with some tb-update?

thx a lot :)

claudia