Forms not sending email notifications

We have two clients who have reported that their forms are no longer sending email notifications.
We have tested these forms by updating workflow emails to be different addresses and it works 100% up until we reinstate the original email address and once again the original email address at which point it fails again.
The emails are not being sent to spam and the email address is working for the client to send/receive emails as usual except for the form.
Steps we have taken to reproduce:

• Update form to submit to my email address - Worked 100%
• Updated form to use different client email address (on same domain) - worked 100%
• Updated client email address to be original address - FAIL
  The instances this has happened on are:
• 652008 - Form 1
• 291964 - Form 5
  For 291964 they resolved this by submitting to a new email address, but the 652008 wish to still use info@.
  Has anyone else reported an issue like this happening as this is a new issue to us and has came up twice in the same week.