This has just happened to me today with emailing AP payment remittance slips.  No issue on the 24th but received errors on all today. 

Same error started happening to us this week. 

Also having this problem while trying to email customer invoices. Started out of nowhere this week.. working fine last week...

Do you guys also have multiple email accounts attached to your outlook. 

I did some experimenting. By removing all the additional email accounts from my Outlook and only keeping the one that my remittances get sent through seemed to have fixed the issue and allowed me to send out remittances and monthly rents. 

I have not attempted to re-add those other accounts I removed and see if it all works again. 

You may wish to see the post KathleenF posted in the Announcements forum this morning.  You can subscribe to that forum so you can have those messages emailed to you.

We removed the other accounts and it worked.  Adding one back in and it failed again.