Outlook 2013 Problems sending large mails

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Senior Member
Outlook version
Outlook 2016 32 bit
Email Account
Office 365 Exchange
Tried sending a few large mails yesterday (1x 4MB, 1x 5MB, 1x 8MB 1x 11MB, 1x 12MB and 1x 13MB)
Got a few return errors.
Then I split up the larger emails into two parts (e.g. 12MB-> 2x 6MB) and tried to resend them.
It took ages... after about 20-25 minutes I got again errors (0x800CCC6C) and (0x800CCC0F).

Under Outlook I also have a gmail account. For specific reasons I don't use that account anymore.
However, I did re-send all mails again, whilst using my gmail account. It took a very short while to
resend them, without issues (am using a fiber 500Mbit/s connection up/down, so speed isn't really
a problem).

Small mails: no issues with Outlook, I think upto 5MB is okay as well, but when it comes to larger
mails, then likely one might hit a problem.
Settings for outgoing mail: smtp-mail.outlook.com, 587 and TLS.

Any solutions for this?

Again, the small stuff : no issue. Probably the outlook smtp server has a problem with large mails?


Thank you Diane,
ah, right, it may have something to do with it...
you mean this : smtp-mail.outlook.com ?
I have no idea about the maximum file size.

OTOH, on the 6MB attachment I got a error reading as follows
"550 5.3.4 Requested action not taken; Mail delivery to EXO failed"

Yes, that (smtp-mail.outlook.com). I thought outlook.com supported larger files tho - 10 GB for free accounts and 20 GB for ad-free. What antivirus/firewall do you use? It could be scanning the outgoing message and slowing it down. Try restarting outlook in safe mode and see if it works better. To open Outlook in Safe mode: Close Outlook then hold Ctrl as you click on the Outlook icon. You'll get a message asking if you want to start in Safe mode. Click Ok.

Do you know if your account is on the new outlook.com server? If so, with outlook 2013, you can use auto account setup to add the account as an exchange account - this will sync calendar & contacts too. This might fix the send problems.
Possibly the most common cause is the message is too large to send because you added a huge attachment to a message.
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