as Email Forwards are "not guanranteed" to be delivered, once they are "out on the Internet."
Email Forwards are "not" real Emails, and so, they are carry a 2nd class rating "out on the Internet."
They may get there, they may get there late, or they may not get there at all ...
Also, the Email Forwards are more liekly to "get blocked" on the receiver's end ( Yahoo account ).
So, please have Your Customer to check their "Bulk Mail" or SPAM and their Settings too, as it may be getting blocked on their end.
Reason : Because the Email Forwards are being forwarded straight from Our Server using the Managerslogin.com Domain, they be getting viewed by Yahoo as "SPAM."
So, please check with the client and see if they have received the Emails to their Yahoo account in some other way ?
If Your Customer wants the Email guaranteed, then please consider "not" using the Email Forwards and using a "Real ( Pop3 ) Email" address such as with a ( Pop3 ) Webmail account.
The Customer can just use the Yahoo address straight up and not forwarded.
Also, It is considered more professional to use a Webmail account that is actually based on and uses the domain name anyway such as, firstname.lastname@example.org