Voicemail issues - not recieving voicemails from Asterisk

Hi, apologies if this has been posted before.
Basically, we are having issues sending out voicemail emails out from the PBX to a btinternet account, we are using the sendmail command but we suspect that there are issues with the email headers forming correctly, and the email being rejected.
This was working fine a few days ago, but cannot think of a reason why this would suddenly stop working… If someone could help, I would really appreciate it, many thanks.