I went to the public TT support page
and input 'Max Upload' one answer was:
What are the restrictions on the TalkTalk Business SMTP outgoing mail servers?
In order to prevent the abuse of our mail relays (and consequent issues with anti-spam blacklisting) we operate certain restrictions on the volume of mail that we will relay.
If you are connecting from within our network (ie from a TalkTalk Business IP address) then these limits are:
Max Recipients/Message: 150
Max Message Size: 32MB (after MIME encoding, in practical terms this is about 20Mb of attachments)
Max Connection per IP: 2
Max Recipients/Hour: 3500
If you are not connecting from within our network (ie using another ISPs connection or roaming) and authenticating with our relay server then the maximum number of recipients per hour is reduced to 250.
Please note that we reserve the right to change these limitations based on operational requirements.
then 'usage limits':
What is my usage quota?
All services have unlimited usage subject to the Fair Usage Policy
Hope this helps.
Market 1 exchange: 565 residential 38 non-residential premises.