Whenever I've seen 1506, it's been caused by some SMTP settings that the SMTP server doesn't like and rejects filemaker.
Do you have an alternate SMTP server to test with, or possibly tweak settings?
SavvyData has a test file they built for testing settings that might be of use to you:
Who is your mail server? I've had occasional problems with Gmail that have no cause, just Gmail.
I work before with the same Computer, the same server, but with fmp130v5 and everything work fine. Now with fmp14 all emailings failed in my sripts. If i send Mail with Outlook, it failed with -1, if i send over smtp it failed with 1506. I don't understand the difference beetween filemaker 13 and 14.