You need to create a custom SPF record as you can only have one. MiaB already has one and your software wants you to use another one, so you need one that combines the information from the two records.
There are spf record generators online, but to be honest, I am not familiar with them or spf records themselves really.
The error makes it look like you may have possibly been entering the record incorrectly … not sure though. (I mean the format you were entering it in with - not the fact that you were trying to enter a second record.)
Alternative possibility - Does your accounting software allow for the invoices to be sent via a third-party SMTP server? If so, you could send the invoices through your MiaB and not have to deal with these custom DNS entries at all…
Ok, I am guessing here … but can you set the email address that the emails will be sent from as something like whatever@billing.mydomain.com? Basically, have the email software send emails as though they are coming from a subdomain? I am thinking that will cause less problems with email delivery from your box itself as you won’t be messing with those subdomain’s dns records … and won’t have to worry about the custom SPF record (I think).
To edit the current spf record, you enter it on the custom dns entries page in the admin area. If the record replaces another record, it will overwrite it.
Yes, you have to edit/add that record on a subdomain by subdomain basis. It seems that wildcard subdomains are not allowed. (The ‘@’ symbol indicates all subdomains)