What version of FMP?
What version of what OS?
maybe you are hitting the size limit of attachments your mail server is allowing you ?
It's a bug in Mail.
If you set the dialog on you can see the email get created, then the attachment pops in a second later.
I am now sending via SMTP rather than email client, but that is in a controlled environment.
I also had the issue sending invoices from MYOB, MYOB patched their program, it waits a couple of seconds between creating and sending the email.
FMP 184.108.40.206 Client (but have tried clients 12, 13, & 14 as well but got same results)
Thanks for bringing this up, totally forgot about checking this. However, once I checked, that isn't the problem either.
To be clear, are the attachments working with smtp?
There are a number of threads about this on the forum and it is a well known issue. So, you are not alone. Clearly an Apple Mail problem in El Capitan.
It doesn't appear (at 10.11.5) that a fix is likely.
We inserted a delay using Applescript which we're rolling out and seems to work (and others have done).
For us, SMTP (which seems to work fine) is not an alternative in all cases as customers want to review and edit the outgoing email.
Well, thankfully the customer was willing to go with the SMTP route. I got it working fine using that.