Email sender sidelined to spam at VM?

I have one particular sender whose emails have recently got bounced to spam at VM mail.

This means they are not getting through when I check emails using any of my three email apps (Thunderbird desktop/android phone email/ipad email.

I have taken to going onto my VM mail to find his emails in spam and flagging them not spam - in which case they then get through to inbox on VM and then all my 3rd party apps... Ive tried putting the senders email address in my VM address book, but this hasnt helped.

Ive got fed up with this so I phoned VM Homeworks for some help - ouch.....Ive now been escalated through four levels - Homeworks agents 1 and then 2, then Gadget Recue, then Cable Care, and now its stuck with FMS(?)  awaiting a ticket response within 5 days. I was on the phone for 2.5 hours this afternoon.

It seems to me the problem is at VM end, because if I log in to VM without even opening my third party apps the emails are in VM spam...however all the support agents seemed obsessed with Thunderbird and kept suggesting I 'spoke to Thunderbird people.'  They also kept wanting me to re generate my 3rd party email passwords......why??

Am I seeing this wrong ?   At no point did it feel like the support people were actually listening to me...they kept asking me questions which suggested they didnt fully understand my set up...I await ticket response atm, feeling frustrated.