The issues you are describing is a known issue for your specific version of ALM. There is knowledge document (QCCR1J96899) regarding this issue and as of current the only way to resolve this issue, other then your suggestion of setting up different temporary email accounts without the digits in the domain, is to the do the following according to Microfocus:
''The only way to go around the problem is to populate the e-mail address through the database USERS table and use SendMail OTA function for delivering mail to these accounts.''
As you have found through your testing the issue is because of the digits within the domain portion of the email address.
Hope this helps,