BT Mail users have been left unable to login to the email service
BT Internet customers are unable to access their email accounts.
The issues first started around 13:30 GMT.
The outages are linked to the current Yahoo Mail outage, which affects customers across Europe and the UK.
BT Internet has reassured customers who have been left unable to login to their emails addresses.
BT Internet has reassured customers affecting by the problems with BT Mail
The official support account tweeted: "Sorry if you're having problems accessing BT Yahoo Mail this afternoon, we're aware of a problem & are working with Yahoo! to fix asap"
Independent website Down Detector, which measures social mentions around a certain topic to track outages across the globe, shows problems with Yahoo Mail throughout the UK and Europe.
Enraged customers have flooded social media to complain about the problems.
"@BTCare is there a problem with BT mail today? Your online chat centre not responding & BT contact numbers all busy?" one user tweeted.
By tracking mentions across social media, DownDetector shows the outage across the UK and Europe
Another posted: "@bt_uk Can you please tell us how long BT Yahoo mail is going to be faulty? It's very inconvenient. Thanks."
"It seems the BT Server to get Mail is currently tempermental…" one BT Mail user tweeted.
According to the latest crowd-sourced statistics from DownDetector, some 44 per cent of users have been left unable to login.
Meanwhile, 29 per cent and 26 per cent respectively are currently unable to receive or read emails on Yahoo Mail.
This latest issue comes at a bad time for Yahoo.
The global technology company admitted a fresh security breach to its systems resulted in the theft of personal information from at least one billion user accounts.
An investigation is still ongoing into the breach, which Yahoo said took place in 2013.
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"Yahoo has identified data security issues concerning certain Yahoo user accounts," the firm said in a statement.
"Yahoo has taken steps to secure user accounts and is working closely with law enforcement.
"As Yahoo previously disclosed in November, law enforcement provided the company with data files that a third party claimed was Yahoo user data.
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"The company analysed this data with the assistance of outside forensic experts and found that it appears to be Yahoo user data.
"Based on further analysis of this data by the forensic experts, Yahoo believes an unauthorised third party, in August 2013, stole data associated with more than one billion user accounts."
Yahoo said that the stolen information includes names, email addresses, telephone numbers, dates of birth, and hashed passwords.
But payment card data or bank account information was not compromised, Yahoo added.
Yahoo also provides email accounts for a number of partners, including BT, Sky UK, Verizon, AT&T, Frontier, Rogers, Spark, and MTS.